The hapten of deoxycorticosterone sulfate was synthesized and linked to the carrier protein through the C-4 position on the steroid nucleus. The antisera raised against this antigen in guinea pigs exhibited high affinity (Ka = 1.86 x 10(9) M-1) and excellent specificity for deoxycorticosterone sulfate. It was found that deoxycorticosterone sulfate can be determined in the range of 50-5000 pg, and this radioimmunoassay in plasma shows a coefficient of variation (CV) less than 10%.