Progesterone secretion by primary cultures of luteinized human granulosa cells was markedly reduced when the cells were incubated in lipoprotein-deficient medium. Addition of LDL, but not HDL3, to cells cultured in lipoprotein-deficient medium stimulated progestin secretion. The effects of LDL were dose-dependent and saturable (Km = 5.5 micrograms LDL protein/ml). LDL also stimulated [3H]-oleate incorporation into cellular sterol esters, with half maximal stimulation occurring at LDL concentrations of 10 micrograms protein/ml. The cultured cells bound and internalized [125I]-LDL in a dose dependent and saturable manner (Km = 5-10 micrograms LDL protein/ml). [125I]-LDL uptake was specific in that unlabeled LDL, but not unlabeled HDL3, competed with labeled LDL for uptake [125I]-HDL3 was also taken up by the cells, but by a lower affinity mechanism. We conclude that luteinized human granulosa cells utilize LDL-carried cholesterol for progestin synthesis, and that LDL is taken up via a specific, high affinity process.