Inclusion Criteria:

* Recipients of an alloSCT for AML or MDS from an HLA-matched donor
* AML/MDS relapsed post-alloSCT with measurable residual disease defined by at least 5% of more myeloblasts based on bone marrow biopsy morphology by pathologist review. Abnormal myeloblasts cannot not exceed 30% overall
* Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) 0-2
* A DLI is available, or the donor is available and agrees to undergo apheresis to collect lymphocytes for infusion
* If salvage therapy for post-alloSCT relapse was received, the therapy is limited to 1 cycle of the following:

1. For hypomethylating agents, venetoclax, and targeted therapies (e.g., tyrosine kinase inhibitors, IDH1/IDH2 inhibitors, or FLT3 inhibitors), the last dose must be \> 2 week prior to the initiation of IFN-γ
2. For cytotoxic chemotherapy agents, the last dose must be \>2 weeks prior to start of treatment for the present study
3. For investigational agents, the last dose must be ≥ 4 weeks or 5 half-lives (whichever is longer) prior to the start of treatment for the present study
* Provision of signed and dated informed consent form
* Stated willingness to comply with all study procedures and availability for the duration of the study
* For female subject, who is \< 55 years old without hysterectomy, oophorectomy or documented menopause, willingness to use two forms of contraception including one form of highly effective contraception (i.e., long-acting reversible contraception, oral contraceptive pills) for the duration of the study
* For male subject, willingness to use highly effective contraception methods including male condoms by male subject and one form of highly effective contraception by his female partner (i.e., long-acting reversible contraception, oral contraceptive pills) for the duration of the study

Exclusion Criteria:

* Primary engraftment failure after alloSCT
* Grade 3 or 4 aGVHD per Mount Sinai Acute GVHD International Consortium (MAGIC) at the time of planned enrollment
* History of grade 4 aGVHD per the MAGIC criteria
* Moderate or severe cGVHD per NIH Consensus Criteria at time of planned enrollment
* Any systemic immunosuppressive medications taken within 2 weeks before the enrollment
* Grade 3 or higher non-hematologic toxicity related to any prior therapy at the time of enrollment
* A contraindication to receive IFN-γ including a known hypersensitivity to IFN-γ, E. coli derived products or any other component of the product
* Positive pregnancy test or currently breastfeeding on Day 1 of study treatment
* Active cardiac arrhythmia not controlled by medical management or current NYHA class II or higher congestive heart failure within 2 months of enrollment unless it was due to a tachyarrhythmia which is under control at the time of enrollment
* Active ischemic heart disease not controlled with medications within 2 months of enrollment
* Acute or chronic pulmonary disease requiring continuous oxygen treatment
* Seizure disorder not controlled by medications within 2 months of enrollment
* AST or ALT \> 5x ULN or total bilirubin \>3x ULN at time of enrollment
* Renal function eGFR \<30 mL/min at time of enrollment using modified Cockcroft-Gault formula
* Body surface area ≤ 1.5 m2 or ≥ 2.5 m2 so as to minimize variation in IFN-γ exposure based on differences in body surface area