The absolute configuration of 2-sec-butyl-4,5-dihydrothiazole (DHT) in urine of adult male mice was determined through chiral trifluoroacetyl derivative capillary chromatography by comparing the retention time with synthetic standards. (S)-DHT was extracted from fresh urine, while neither (R)-DHT nor the racemization of (S)-DHT were detected. We can conclude that DHT in urine possesses the S configuration, although we cannot exclude a minor component in the R configuration. (S)-DHT was then characterized for binding to the complex of major urinary proteins of male mouse urine (MUP) and for a behavioral response, the competitive scent marking behavior (countermarking). The binding constant of (S)-DHT to MUP (determined by competitive displacement) was 8.2 +/- 0.6 microM (mean +/- SD) and was 10.5 +/- 0.6 microM for R-DHT, thus excluding a relevant difference in binding. (S)-DHT modified countermarking in a peculiar way. Male mice were slow in countermarking urinary spots streaked 2 days earlier and on top of which (S)-DHT was added shortly before the test. This response was not seen when adding (S)-DHT to freshly streaked urinary spots or to clean paper. Unlike (S)-DHT, (R)-DHT prompted countermarking rather than delaying it. We can further conclude that (S)-DHT in male mouse urine is an aversive chemosignal for countermarking.