The effect of Colcemid on the frequency and type of chromosomal abnormalities in human sperm was investigated. Using the human sperm and zona-free hamster egg fusion technique, penetrated eggs were cultured in the presence or absence of Colcemid. We used two different times of Colcemid treatment: standard Colcemid treatment (Colcemid-5 h) or long Colcemid treatment (Colcemid-17 h). Each Colcemid series had its own control series without Colcemid, thus ensuring that Colcemid was the only significant variable. A total of 771 sperm karyotypes from one normal donor was analyzed: 286 in the Colcemid-5 h series, 262 in the Colcemid-17 h series, and 223 in the two control series. In both Colcemid series there was a significant increase in the frequencies of hypohaploidy vs. hyperhaploidy (9.4% and 7.3% vs. 2.4% and 1.1%, for the Colcemid-5 h and Colcemid-17 h series, respectively), in contrast to those obtained in the control series, in which the frequencies of hypohaploidy and hyperhaploidy were close to the 1:1 relationship (4.9% vs. 4.0%) expected from nondisjunction. There was a significant increase in the frequency of structural abnormalities in both Colcemid series (16.1% and 14.5 % for the Colcemid-5 h and Colcemid-17 h series, respectively) compared to the control series (6.3%). These results suggest that Colcemid significantly increases the frequency of hypohaploidy and unstable structural aberrations in human sperm. © 1996 S. Karger AG, Basel.