Using the methylase Sssl enzyme, we have analyzed the degree of in situ methylation of human sperm pronuclear chromosomes obtained by fertilizing hamster oocytes with human sperm. Untreated (control) sperm chromosome complements showed a higher degree of in situ methylation, compared to sperm complements previously treated with 5-azadeoxycytidine or lymphocyte chromosomes. This indicates that human sperm pronuclear chromosomes have a lower degree of genomic methylation compared to that of other somatic cells. The similarity in the degree of in situ methylation of the euchromatic and heterochromatic regions of chromosomes 1,9, 15, and 16 and the Y chromosome in human sperm does not support the existence of a possible correlation between hypomethylation and heterochromatin decondensation. © 1997 S. Karger AG, Basel.