Repetitive sequences of the Xho1 family comprise about 70% of the W chromosome in chickens, are considered to be restricted to this sex chromosome and are used in sex diagnosis. This study investigated the reliability of sex diagnosis on small cell numbers. Fluorescent in situ hybridizations (FISH) with the Xho1 repeat sequence were performed on metaphases and signals were detected on the W chromosome and on some autosomes. Differences in signal intensity suggested that the number of repeats ranged from several thousand on the W chromosome to only a few on the autosomes. These results confirm the report from Clinton (1994) of minor male products in sex specific PCR of the Xho1 repeat. We adopted his PCR protocol to analyze small numbers of cells. FISH on blastomeres and on blood cells from E3 to E11 embryos were compared to PCR results. The FISH procedure as well as the PCR protocol are suitable for sexing. For single cells FISH was slightly more consistent.