The premise that one manifestation of the nexus between Sertoli cells and germ cells may be an orderly and sequential change in their protein profiles has been examined in relation to the ontogeny of spermatogenesis in the colony-bred albino rat. Viable "Sertoli cell-germ cell associations"1 isolated from the testes of animals of defined postnatal age and incubated in a n appropriate medium were separated into a Sertoli cell and a germ cell fraction and processed for analysis by sodium dodecyl sulfate gradient gel electrophoresis. The resulting stained bands were "mapped" and assigned relative mobility values by comparison with standard marker proteins. This enabled identification by serial number of individual bands from an overall total of 163. For purposes of detailed analysis, they were classified into high, medium-high, medium, and low molecular weight bands. Two major categories were delineated: 1) those associated uniquely with a specified day of ontogeny and 2) those appearing intermittently. Significantly enough, not one of the bands was encountered on all days examined. The relevance of the patterns observed to the possible exchange of "information" between Sertoli cells and germ cells during spermatogenesis is mooted. 'The term "Sertoli cell-germ cell associations" is used to designate the groups of Sertoli cells together with the associated germ cells (in other words, a part of seminiferous epitheliumj that are recovered after enzymatic removal of tunica propria from the seminiferous tubule fragments (Welsh and Wiebe, '75; Meenakumari and Duraiswami, '86b).