PRL significantly increases CPT1 enzyme activity in MDA-MB-231 breast cancer cells. Enzymatic analysis of cell lysates harvested 24 hr following culture in the absence and presence of varying doses of recombinant human PRL (0, 25, 100, and 500 ng/ml). 100 ng/ml of PRL slightly increased CPT1 enzyme activity in A) 184B5 cells by 8.3% and in B) MCF-7 cells by 8.5%. C) CPT1 enzyme activity was significantly enhanced in MDA-MB-231 breast cancer cells treated with 100 and 500 ng/ml of PRL, resulting in 21.2% and 18.8% increases, respectively (p < 0.02). Results are presented as fold of enzyme activity (nmol CoA-SH released/min/mg protein) relative to untreated cells and represent the mean ± SEM of three independent experiments. Different letters represent statistical differences between groups.