Nuclei of mouse Sertoli cells were examined on air-dried toluidine blue-stained preparations to analyse factors influencing the aggregation of heterochromatin into chromocentres. For the CBA strain males tested at 1.3, 2, 3, 6, 9 and 12 months of age, mean numbers of heterochromatin bodies were 4.8, 2.4, 2.1, 1.8, 1.6 and 1. 4, respectively; two chromocentres predominated from 2 to 9 months of age. In the KE strain, heterochromatin aggregation was significantly accelerated; nuclei containing only one chromocentre were predominant from 6 months. The number of chromocentres did not change with the stages of the seminiferous cycle, and after 1 month of cryptorchid condition. Cryptorchidism resulted in disruption of spermatogenesis and Sertoli cell dysfunction, as demonstrated by the lack of immunohistochemically detectable androgen receptors. The difference in the number of chromocentres between KE and CBA Sertoli cells persisted after 3 days of in vitro culture, but unidentified cells with numerous chromatin bodies were also observed. Testing recombinant inbred strains indicates that at least two genes are involved in the difference in the number of chromocentres between progenitor KE and CBA strains; however, no correlations were found with 15 marker loci or with parameters linked to reproduction. Of the eight strains tested, AKR and C3H showed a 'CBA-like' chromocentre pattern; C57BL, B10.BR, B10.BR-Y(del) and KP were 'KE-like'; and BALB/c and DBA/2 were intermediate. The results showed that centromere aggregation in the Sertoli cell progresses throughout the life of a male in a strain specific manner; however, its functional significance remains unknown.