In our laboratory we encountered problems interpreting the standard Friberg test. After an incubation period of four hours we could not interpret the test in 52 per cent of cases due to a high number of immobile spermatozoa and debris in the supernatant used as antigen in this test. By shortening the incubation period to two hours we could objectively interpret 76 per cent of cases. The applicability of the test was further improved by modifying the preparation of the antigen. Instead of using the supernatant of diluted semen, we "filtered" the motile sperm by means of the sperm penetration meter of Kremer. The capillary tube of the penetration meter was filled with virgin serum and incubated for one hour. After a four hour incubation this test could be applied in 80 per cent of cases, and after a two hour incubation period in all the cases. The reliability of this test was compared with the standard Friberg, the Kibrick and the Franklin-Dukes tests. The modifications did not alter the reliability of the test.