The present study aims to identify a chains of type IV collagen in the basement membrane of the mouse ovarian follicle and examine their changes during follicular development using immunofluorescence microscopy with specific monoclonal antibodies. The basement membrane of the serous mesothelium enveloping the ovary contained all a chains of type IV collagen, alpha 1(IV) through alpha 6(IV) chains. Primordial follicles showed a distinct immunoreactivity against all six a chains in their basement membranes. Immunolabeling for alpha 3(IV) and alpha 4(IV) chains was almost eliminated in the primary follicles. In basement membranes of secondary and Graafian follicles, the immunofluorescent reaction of a3(IV) and a4(IV) chains disappeared in Graafian follicles, a partial reduction in fluorescent immunostaining intensity to alpha 5(IV) and alpha 6(IV) chains was observed; only alpha 1(IV) and alpha 2(IV) chains were not degraded throughout follicular development. On atretic follicles, in addition to alpha 1(IV) and a2(IV) chains, alpha 3(IV), alpha 4(IV), alpha 5(IV) and alpha 6(IV) chains frequently persisted. A basement membrane-like matrix within the follicular granulosa cell layer, such as the focimatrix (focal intraepithelial matrix) and/or Call-Exner body, was also recognized in mouse secondary and Graafian follicles and contained alpha 1(IV), alpha 2(IV), alpha 5(IV) and alpha 6(IV) chains but not alpha 3(IV) and alpha 4(IV) chains. We expect that the decrease in alpha(IV) chains prompts follicular development and is a prerequisite condition for follicular maturation.