This report describes the findings of an evaluation of the validity of the new, highly automated Serono SR1 enzyme immunoassay for prostate specific antigen performed at three European sites. Within-run precision was below 5.5% at the clinically important decision point of 4 micrograms/l, less than 10% below 3 micrograms/l and less than 5% above 4 micrograms/l. Between-run precision was below 12% at each site and between-site precision was less than 14%. Dilution series were shown to be parallel to the standard curve on 12 samples with neat prostate specific antigen concentrations ranging from 20 to 88 micrograms/l (linear regression slopes and correlation coefficients of 1.00 to 1.06 and 0.972 to 1.000 respectively). Comparisons with Hybritech Tandem-E and -R and Roche Cobas Core prostate specific antigen methods used routinely at the three sites were good (linear regression slopes 1.04 to 1.13, correlation coefficients 0.96 to 0.99), but results were significantly lower than the Ciba Corning ACS180 (slope 0.66, correlation coefficient 0.99). The assay presents a precise, robust and valid diagnostic tool offering additional advantages of non-isotope automation.