Specific antibodies against diethylstibestrol (DES) were obtained from one ewe immunized against DES-Succinate-human serum albumin conjugate. Using tritiated DES as a tracer, a radioimmunoassay for DES was developed. The senstivity of the standard curve was between 0.25 and 0.5 ng, using a mass of tracer of about 0.5 ng. The antiserum did not cross react with any naturally occurring steroid tested. After oral administration of 10 mgm of DES to a male subject, plasma DES measured after ether extraction reached peak levels of 18.4 and 18.6 ng/ml of plasma 1 and 2 hours later. Plasma DES was still detectable 10 hours after administration. Proof of the validity of this assay for the measurement of DES in cattle would require testing the interference of many plant nonsteroidal estrogen chemically related to DES.