Background: The first rituximab biosimilar approved in the US, rituximab-abbs, is a CD20-directed monoclonal antibody that is expected to significantly reduce drug acquisition costs. This economic analysis evaluated budgetary impact of BSA-based rituximab-abbs (IV-R-BIOSIM) vs flat-fixed rituximab/hyaluronidase human subcutaneous injection (SC-R) dosing in NHL (diffuse large B-cell lymphoma [DLBCL], follicular lymphoma [FL]) and CLL. The objective was to project incremental cost differences per patient between IV-R-BIOSIM and SC-R from a US healthcare insured (Medicare) population. Methods: An illustrative BIM estimated 1-year costs for IV-R-BIOSIM and SC-R. The model assumed equal efficacy and safety between products. Values for epidemiology, market share distribution, drug dosing, administration, and costs were derived from scientific literature, product labels, and publicly available cost resources. Costs for the first infused rituximab (IV-R) dose were excluded for appropriate comparison. IV-R-BIOSIM doses used BSAs of 1.6 m², 1.8 m^2, infusion duration was 3 hours. Annual dose counts of IV-R-BIOSIM or SC-R were: 9 untreated FL with maintenance; 7 untreated FL (without maintenance), relapsed/refractory FL, or untreated DLBCL; 5 CLL. Drug acquisition and administration costs were from 2020 Average Sales Price pricing file and Centers for Medicare and Medicaid Services Physician Fee Schedule. Patient cost share was 2020 Medicare Part B 20% cost-share for office visits and drugs. A scenario analysis was also performed to estimate FL maintenance costs for 2-year dosing. Results: Estimated 1-year savings with IV-R-BIOSIM for 1.8 m² BSA dosing were $1,067–$6,893 with variability between indications (Table). For 1.6 m² BSA dosing, estimated 1-year savings with IV-R-BIOSIM were $3,819–$10,856. Estimated 2-year savings with IV-R-BIOSIM for FL maintenance dosing were $9,191 for 1.8 m² BSA dosing and $14,475 for 1.6 m² BSA dosing. Savings of up to $1,900 were seen for higher-than-average BSA dosing, regardless of regimen. Conclusions: These findings demonstrate the potential economic benefits of replacing a proportion of SC-R use with BSA-based IV-R-BIOSIM from a US payer perspective, especially when lower BSA dosing is used. Savings are driven by drug costs and may increase with IV-R-BIOSIM as patient BSA decreases due to static costs with SC-R doses. These data also suggest that drug wastage may occur with SC-R in lower BSA patients.