The presence of receptors for oestradiol-17 beta and 5 alpha-dihydrotestosterone (5 alpha-DHT) in the human monocytic leukaemia cell line J111 and rat peritoneal macrophages was investigated using whole-cell assays. For both cell types, high-affinity binding species for oestrogen were detected, whereas no indication of specific binding was observed for 5 alpha-DHT. Analysis of the data according to Scatchard showed curved lines, indicating the presence of two different oestrogen-binding species. The dissociation constant (Kd) values for the receptors of the rat peritoneal macrophages were calculated to be 1.4 x 10(-10) M and 3.3 x 10(-9) M, while for the J111 cells, the Kd values were 8.7 x 10(-11) M and 2.5 x 10(-9) M. Sucrose-gradient ultracentrifugation identified one oestrogen-binding species of 7.1S. The receptors had a relatively high affinity for diethylstilboestrol (DES) but did not bind to a monoclonal antibody specific for the classical oestrogen receptor, suggesting that oestrogen receptors in macrophages could be of a different type.