Steroid derivatives containing histidine methyl ester (HME), instead of histamine, were prepared by mixed anhydride coupling. The derivatives were crystalline, and when labelled in microgram quantities by using Iodo-gen (exposure time 1 h) the yield of the immunoreactive fraction was 40-50%. The products were similar in immunoreactivity and stability to the known histamine derivatives. Assay parameters obtained with HME-derivatives were compared with those obtained with tritiated steroids and with analogous TME-derivatives. A heterologous assay of progesterone (3 antisera against 12 alpha-succinyl-BSA, and methylsuccinyl derivatives for labelling substituted at 11 alpha-position), and a homologous assay of cortisol (4 antisera against 21-succinyl-BSA, and 21-carbonyl-derivatives for labelling) were studied. The HME-derivatives produced logit-log curves with slopes comparable to those in tritium-based assays. The sensitivity, as expressed by ED50 values, was by 66% higher than in tritium assays, and by 50% higher than for the tyramine derivative in a similar heterologous assay. The HME-based assay for cortisol was by 26% less sensitive compared to tritium, but several times more sensitive than in case of analogous TME-based assay.