In vitro work suggests that conjugated linoleic acid (CLA) isomers (c9,t11 and t10,c12) are cytotoxic to human breast cancer cells, however the mechanism remains unknown. Using human MCF-7 breast cancer cells, we examined the effects of c9,t11 and t10,c12 CLA compared to oleic acid (OA), linoleic acid (LA), or untreated cells on cell membrane phospholipid composition, cell survival, and the insulin-like growth factor-I (IGF-I) and the downstream insulin receptor substrate-1 (IRS-1). Both CLA isomers were incorporated into membrane phospholipids (p < 0.05). Compared to untreated cells, c9,t11 or t10,c12 CLA significantly reduced the metabolic activity of IGF-I stimulated MCF-7 cells, increased lactate dehydrogenase (LDH) release, and decreased cellular concentrations of the IGF-I receptor (IGF-IR) and insulin receptor substrate-1 (p < 0.05). Incubation with t10,c12 CLA also reduced the levels of phosphorylated IGF-1R. The effects on all of these measures were greater (p < 0.05) for t10,c12 CLA compared to c9,t11 CLA. There were few differences between LA-treated and c9,t11 CLA-treated cells, whereas cellular metabolic activity, LDH release, and IGF-IR concentrations differed between t10,c12 CLA-treated and LA-treated cells (p < 0.05). OA stimulated growth compared to the untreated condition (p < 0.05). In summary, this study demonstrated that the t10,c12 CLA isomer inhibits growth of MCF-7 cells and suggested that this may be mediated through incorporation into cellular phospholipids and interference with the function of IGF-I and related signaling proteins.