To determine whether luteotrophic activity is present in the bovine placental granules, fetal cotyledons from fetuses of 50-100 days of gestation were used. Enriched granules were prepared using a Percoll gradient. Active substances were obtained from the granules by freeze-thawing. The extracts thus obtained were then eluted on a Sephacryl S-300 column. The resultant fractions were then analysed by (1) a radioreceptor assay for hCG-like substances and (2) a bioassay using progesterone production by bovine luteal cells. There were two peaks of activity, one indicative of a high molecular weight substance and the second of a low molecular weight substance. Higher molecular weight substances were eliminated by using acidic extracts. The low molecular weight fraction was further analysed using reverse phase h.p.l.c. (acetonitrile:water gradient). The elution of this substance at 45% acetonitrile resulted in a 100-fold increase in luteotrophic activity in the bioassay compared to the Sephacryl fraction. The small molecular weight substance is heat-stable and not extracted to the organic phase when partitioned between methanol and chloroform.