The effects of dietary soybean oil (SBO) concentration (5 vs. 20% by weight) on mammary tumorigenesis, mitogen-induced blastogenesis, cell-mediated cytotoxicity and serum and lymphocyte fatty acid composition were studied in C3H/OUJ female mice. Weanling mice fed 20% SBO for 9 mo had a higher incidence (89 vs. 65%) and greater average size (2.9 vs. 1.9 g) of mammary tumor virus type S breast tumors than mice fed 5% SBO. The response of isolated splenocytes to the T-cell mitogens concanavalin A and phytohemagglutinin was 20-25% lower in mice fed 20% SBO than in mice fed 5% SBO for 20 wk. There was no effect of SBO concentration on the splenocyte response to lipopolysaccharide E55:B5, a B-cell mitogen, or to pokeweed mitogen, and B- and T-cell mitogen. Cell-mediated cytotoxicity to allogenic P815 tumor cells was 20% lower in mice fed 20% SBO than in mice fed 5% SBO for 12 wk. The lower cell-mediated immunity associated with 20% SBO was not due to changes in the fatty acid composition of the two major splenocyte membrane phospholipids, phosphatidylcholine and phosphatidylethanolamine. However, serum levels of linoleic acid were higher in mice fed 20% SBO. Raising dietary SBO from 5 to 20% by weight was associated with increased mammary tumorigenesis, reduced T-cell blastogenesis and lower cell-mediated cytotoxicity.