This report describes the use of 21-, 16 alpha- and 11 alpha -[2'-3H]bromoacetoxyprogesterone as affinity labels to characterize the human uterine progesterone receptor (HPR). These three derivatives can bind to and displace progesterone bound to the HPR. This affinity labelling was inhibited by an excess of radioinert progesterone and could not be demonstrated if bovine serum albumin was used in place of the HPR. Bromoacetic acid alone did not affinity label the HPR. Polyacrylamide gel electrophoresis under denaturing conditions showed that all three derivatives bound to a 45,000 molecular weight protein.