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Summary. Corpora lutea from cyclic ewes were dissociated by collagenase digestion and trypsin/EGTA treatment. Enriched fractions of endothelial cells, small luteal cells and large luteal cells were prepared on a stepped gradient of Ficoll 400. Progesterone was measured by radioimmunoassay and the results corrected so that progesterone production by each cell type could be determined. Endothelial cells did not produce significant amounts of progesterone, with or without LH stimulation, and endothelial cell contamination of small and large luteal cell fractions did not influence progesterone production by these fractions. Mean ± s.e.m. basal progesterone production (n = 10) by large luteal cells was greater (P < 0·001) on a per cell basis than that by small luteal cells (1·16 ± 0·16 compared with 0·25 ± 0·06 pg/h/cell). However LH, which stimulated a maximal 3–4-fold increase in progesterone production by small luteal cells (LH ED50 = 0·14 ng/ml), had no significant effect on production by large luteal cells, when contamination by small luteal cells was taken into account. The response of small luteal cells was specific to LH, other hormones having had no significant effect.
Basal progesterone production by small luteal cells (0·12 ± 0·03 fg/h/µm3) calculated per unit volume of cell was not significantly different from that of large luteal cells (0·17 ± 0·02 fg/h/µm3). After LH stimulation, small luteal cells produced more progesterone than did large luteal cells (0·40 ± 0·09 compared with 0·18 ± 0·03 fg/h/µm3) (P < 0·05). When the amounts of progesterone produced per cell were multiplied by the absolute numbers of large luteal (1 x 107) and small luteal (5 x 107) cells in the intact corpus luteum, basal progesterone production by large luteal cells (11·6 ± 1·6 µg/h) was similar to that by small luteal cells (12·3 ± 3·0 µg/h). However, under LH stimulation, progesterone production by the small luteal cell type (39·9 ± 9·5 µg/h) was 
3 times greater than that by the large luteal cell type (12·3 ± 1·6 µg/h) (P < 0·05).
We therefore conclude that small luteal cells may be the principal source of luteal progesterone production in the sheep.
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