Radioimmunoassay procedures have been adapted for the assay of progesterone, 17-hydroxyprogesterone, estradiol-17beta, and prostaglandin F in human corpus luteum. The method utilises a single homogenisation and extraction of the tissue followed by fractionation of the steroids on alumina, and separation of the prostaglandins of the F series from the E and A series on silica gel, prior to radioimmunoassay. An attempt has been made to validate the method for the progestins by comparison with results after fractionation of the progestins on Sephadex LH-20, for estradiol-17beta by comparison with values obtained with competitive protein-binding, and for prostaglandin F by comparison with values after additional purification. The results showed that peak concentrations of the three steroids in corpora lutea from women during the luteal phase of the menstrual cycle were comparable to those found in corpora lutea from women in early pregnancy. However, in six out of fourteen corpora lutea from non-pregnant women, prostaglandin F levels were higher than those found in corpora lutea from seven women in early pregnancy, i.e. 13-46 ng/g compared with 1-7 ng/g. Of the above six corpora lutea, four were on days 23-25 of the cycle, at a time when luteolysis would be commencing. The results in this paper support the conclusion that the corpus luteum is a major site of synthesis of the three steroids examined, although the site of synthesis of prostaglandin F is still equivocal.