Prostatic acid phosphatase (PAP) was measured in 70 patients with stage D prostate cancer under different modalities of treatment. PAP was determined by radioimmunoassay (RIA), counter immunoelectrophoresis (CIEP), and enzymatic method using alpha-naphthyl phosphate to compare the usefulness of the three methods in follow-up and assessing the clinical status of stage D prostate cancer. In the regressive state (29 patients), RIA and enzymatic methods correlated well; both gave 17% of abnormal results with a mean value of 4.7 +/- 4.6 and 3 +/- 1.7. Also, in the progressive state (17 patients) the two methods showed similar percentages of abnormal results with a mean value of 40 +/- 38 and 19 +/- 17 for RIA and enzymatic method, respectively. There was greater variability in the stable group owing to the difference in the tumour load. Again the two methods correlated well regarding their diagnostic sensitivity and specificity as a parameter for assessing the clinical response. CIEP, used as a qualitative method, showed more positive than negative results and did not correlate with the clinical state. We feel that the conventional enzymatic method is adequate for follow-up and assessing clinical state of stage D prostate cancer.