We did a comparative analysis of the physiological and analytical properties of prostate-specific antigen (PSA), acid phosphatase (ACP; EC 3.1.32) activity, and acid phosphatase antigen (PAP) in serum. The PSA assay is sensitive to 0.2 microgram/L and demonstrates good linearity (y = 1.01x + 0.74). The CV was 3.9% at 40 micrograms/L, 8.0% at 3.1 micrograms/L. PSA and PAP are less stable at 4 degrees C than at -20 degrees C. Serum PAP and ACP concentrations showed large intra-individual fluctuations (average CVs of 22% and 24%, respectively), which were not observed with PSA measurements (average CV 6.2%). We saw significant correlation with the magnitude of physiological change when analytes were compared for serially collected split samples [y(PSA) = 0.14x(PAP) + 0.00, r = 0.767], which indicates that a common factor is influencing this variation. The excellent analytical performance, tissue specificity, and small degree of intra-individual variance are characteristics that favor the measurement of PSA in serum for monitoring patients with prostatic cancer.