The differential capacity for steroid synthesis of human luteal cell subpopulations was investigated in a well defined cell culture system. Corpora lutea were enzymatically dissociated, and the two cell types were obtained by a discontinuous Percoll gradient. Both cell types were cultured for 24 h with dibutyryl cAMP (1 mM), oestradiol (2.5 microM) and testosterone (1 microM). Steroid production was measured in the culture media and aromatase activity for both cell type subpopulations was also determined. Basal production of progesterone, oestradiol and testosterone was significantly greater in large cells than that in small cells (P < 0.05). Nevertheless, a greater response of small cells to several in-vitro treatments was observed. Thus, synthesis of progesterone, oestradiol and testosterone was significantly stimulated in these cells (P < 0.05) by dibutyryl cAMP. Interestingly, a 3.3-fold increase of progesterone production was also observed in the large luteal cell subpopulation. When oestradiol was added to the culture media, a 36% decrease of progesterone production (P < 0.05) by small cells was obtained, while progesterone synthesis by large cells was not significantly affected. Testosterone treatment of cells enhanced oestradiol production by both cell subtypes (P < 0.05), although the stimulatory action was greater in the small cell cultures (5.9-fold). These data indicate that the steroidogenic activity of the small cell subpopulation is highly dependent on endocrine and paracrine stimulatory mechanisms, while large cells possess a greater intrinsic steroidogenic capacity.