Criteria for Pre-Screening Blood Draw

* Newly diagnosed with IgG or IgA multiple myeloma (either untreated or receiving first line therapy).
* Potentially eligible for autologous hematopoietic cell transplant (with or without tandem transplant) for frontline therapy.
* At least 18 years of age.
* Ability to understand and willingness to sign an IRB approved written informed consent document. (Legally authorized representatives may sign and give informed consent on behalf of study participants.)

Inclusion Criteria:

* Received autologous hematopoietic cell transplantation (with or without tandem transplant) as part of frontline therapy for newly diagnosed IgG or IgA multiple myeloma.
* 80 to 140 days after AHCT with measurable disease at Day +100 landmark assessment, defined as:

* MRD-positive by clonoSEQ at \> 10-6 myeloma cells/cell and/or
* Achieved less than CR
* ECOG performance status ≤ 2
* Adequate bone marrow and organ function within 28 days prior to start of treatment as defined below:

* Platelets ≥ 25 k/cumm
* Absolute neutrophil count ≥ 1.0 k/cumm; given the well-documented phenomena of lower baseline neutrophil count in patients who identify as Black or African-American (i.e. Duffy antigen null neutrophil counts, previously Benign Ethnic Neutropenia), patients who identify as Black or African American and have a stable ANC of 0.5 - 1.0 k/cumm post-AHCT may be enrolled at the discretion of the treating physician.
* Hemoglobin ≥ 8 g/dL without the use of growth factors or transfusion for at least 2 weeks.
* Total bilirubin ≤ 2 × upper limit of normal (ULN; ≤ 3 x ULN if documented Gilbert's syndrome)
* Aspartate aminotransferase and alanine aminotransferase ≤ 2.5 × ULN
* Creatinine clearance ≥ 30 ml/min. Patients with stable renal function and creatinine clearance of 15 - 30 ml/min may be enrolled on a case-by-case basis after discussion with the principal investigator.
* The effects of elranatamab on the developing human fetus are unknown. For this reason, women of childbearing potential and men must agree to use adequate contraception prior to study entry, for the duration of study participation, and for 5 months after end of treatment. Should a woman become pregnant or suspect she is pregnant while participating in this study, or should a man suspect he has fathered a child, s/he must inform her treating physician immediately.

Exclusion Criteria:

* Inability to identify both an EasyM clonotype and clonoSEQ ID.
* Received \>28 days of standard-of-care maintenance treatment following AHCT. A ≥14 day washout of standard-of-care maintenance treatment is required prior to receiving the first dose of elranatamab.
* A history of other malignancy with the exception of non-melanoma skin cancers, low or very low risk prostate cancer by NCCN criteria status post definitive therapy or currently on active surveillance, and malignancies for which all treatment was completed at least 2 years before registration and the patient has no evidence of disease.
* Currently receiving any other investigational agents.
* Prior BCMA-based treatment.
* Presence of light chain or non-secretory disease.
* CNS involvement of disease.
* A history of allergic reactions attributed to compounds of similar chemical or biologic composition to elranatamab or other agents used in the study.
* Uncontrolled intercurrent illness including, but not limited to, plasma cell leukemia, POEMS syndrome, systemic amyloidosis, ongoing or active infection (bacterial, fungal, or viral).
* Pregnant and/or breastfeeding. Women of childbearing potential must have a negative serum pregnancy test within 28 days of study entry.
* HIV-infected if not on effective anti-retroviral therapy with undetectable viral load for 6 months. Patients with HIV who are receiving effective anti-retroviral therapy and have had an undetectable viral load for at least 6 months are eligible. HIV testing not required in the absence of known history of infection.