The distribution of anti-MLH1 (MutL homologue 1) antibody labelling wa
s studied in human prophase meiocytes. A labelling pattern consisting
of distinct foci, always associated with the synaptonemal complex (SC)
and never in closely juxtaposed pairs, was observed. Comparison of th
e number and general positions of autosomal foci with previous studies
of the number and positions of autosomal chiasmata indicates that the
anti-MLH1 antibody marks sites of crossing over in human pachytene sp
ermatocytes. A mean number of 50.9 autosomal foci was observed from 46
human pachytene spermatocytes corresponding to a genetic length of 25
45 cM. Division of these spermatocytes into sub-stages revealed that t
he number of foci remains stable throughout pachytene. A focus was fou
nd on the XY bivalent in 56.5% of the nuclei. The presence or absence
of foci from the XY bivalent could not be correlated to pachytene sub-
stage.