In situ hybridization to human meiotic metaphase I (MI) preparations, using the labeled minisatellite core sequence λ33.15, showed clustering of autoradiographic grains principally at or around chiasmata, autosomal sites where crossing-over had occurred. For the XY bivalent, the pairing region formed between the terminal regions of the two short arms (Xpter-Ypter), was also a principal site of labeling; in addition, the terminal region of the X long arm (Xqter) was labeled. Control experiments using a member of the human Alu family of dispersed repeated DNA sequences showed a much more randomized grain distribution, with clustering over chiasmata being far less obvious. The data provide support for the suggestion that polymorphic minisatellite regions within the human genome might play a significant role in pairing and/or recombination