The correlation between tumor volume of untreated tumor-bearing nude mice and serum concentration of prostatic acid phosphatase (PAP/RIA) was studied in the hormone-dependent serially transplantable human prostatic tumor models PC-82 and PC-EW. The normal serum level of PAP in control male nude mice without tumor was found to be 0.9 +/- 0.3 ng/ml. Elevated PAP serum concentrations were never found in animals without tumor (a highly specific diagnostic technique). A close correlation was observed between the concentration of PAP in the serum (range 0.3 to 154 ng/ml) and the tumor volume (range 10.0 to 6,530 mm3) of 104 untreated mice bearing a PC-82 or PC-EW human prostatic tumor. This correlation was comparable in both tumor lines (p less than 0.001). The positive effect of endocrine manipulation which resulted in tumor diameter decrease or growth arrest with regressive histological patterns, showed the normal PAP serum level, too. After successful treatment PAP was found to be normal, independent from the residual tumor mass. By contrast, in the event of only retarded tumor growth, the PAP level still correlated with the tumor burden.