Incorporation of interleukin-2 (IL-2) into multilamellar vesicles, IL-2 liposomes, significantly modifies the biodistribution and prolongs the clearance of the cytokine. The rat was used to evaluate and characterize subacute toxicity and short-term pathologic effects of intravenous (i.v.) IL-2 liposomes. Once daily i.v. IL-2 liposomes in doses of 0.3 × 106u/kg, 1.5 × 106u/kg, 4.0 × 106u/kg, and 20 × 106u/kg were given to rats for 15 days; control rats received i.v. empty liposomes or buffered saline. Hematologic parameters, serum chemistry, and gross pathologic and histopathologic findings were recorded. Only at the highest dose of IL-2 liposomes were effects consistently severe enough to produce weight loss, decreased food consumption, hepatitis, anemia, and death. At this dose females were more susceptible to mortality than males; only 1 of 5 females survived compared to 4 of 5 males. Peripheral blood leukocyte increases after IL-2 liposomes were mainly associated with lymphocytosis with only minor changes in neutrophils, monocytes, and eosinophils. The marrow had significant dose-related hematopathologic findings limited to decreased percentages of lymphocytes and increased myeloid:erythroid (M:E) ratio in the 20 × 106u/kg group. No increases in marrow eosinophils or eosinophilic precursors were demonstrated. The major toxic effects of IL-2 liposomes in rats were hepatic. Dose-related elevations in liver enzyme serum chemistries (aspartate aminotransferase [AST], alanine aminotransferase [ALT], bilirubin, γ-glutamyl transferase [GGT], and alkaline phosphatase) were seen. Occasional rats had evidence of bridging fibrosis in addition to centrilobular and periportal lymphocytic and eosinophilic infiltration of the liver. Pulmonary histopathology was characterized by peribronchial and perivascular lymphoid and eosinophilic infiltration with minimal or no septa1 involvement. In summary, the spectrum of pathologic and toxicologic changes in rats receiving once daily IL-2 liposomes are similar to those previously reported in rats given the free cytokine twice daily, but occur at reduced doses with less lung pathology. Thus, compared to free cytokine i.v. IL-2 liposomes in rats have increased potency and decreased pulmonary toxicity. © 1992 Wiley-Liss, Inc.