We tested the performance of the new immunochemical membrane test BioSign PSA for detecting serum concentrations of prostate-specific antigen > 4 micrograms/l. Measurements were simultaneously performed on 161 serum samples, using the strip test and three quantitative assays of prostate specific antigen (Tandem-E, IMx, LIA-mat). Based on the upper reference limit of 4 micrograms/l, 11-37% of the results obtained with the BioSign test were false-negative and 4-15% were false-positive, compared with the data of the three quantitative assays. It is concluded that the BioSign test in its present form does not satisfactorily differentiate between prostate-specific antigen concentrations above and below 4 micrograms/l, but an improved version of the test would be useful.