The levels of prostatic specific antigen (PSA) and prostatic acid phosphatase (PAP), both measured by radioimmunoassay, and two cancer indices given by the ratio of serum alpha-1 acid glycoprotein to prealbumin (AGP:Palb) and alpha-1 acid glycoprotein to alpha-2 HS globulin (AGP:HS) were evaluated as markers to assess the response of prostatic cancer to treatment with Zoladex, an LH-RH agonist. A rise in PSA and PAP occurred in 8/65 patients (12%) during the initial induction phase. In metastatic disease prior to treatment none of these indices was significantly different between patients who attained a sustained response and those whose response was nil or only transient. Responders and non-responders could, however, be distinguished by the levels of various analytes after treatment. At 6 months the median PSA in those who responded was 2.5 ng/ml compared with 51.5 ng/ml in the non-responders. At 12 months the figures were 3.0 and 155 ng/ml respectively. The corresponding median PAP levels were 1.4 and 19 ng/ml at 6 months and 1.3 and 18 ng/ml at 12 months. The AGP:Palb ratio was also significantly different in these two groups at 6 and 12 months. PSA appears to be the most sensitive indicator of the response to treatment. The likelihood of obtaining a prolonged clinical response can be assessed within 6 months of the start of treatment.