The present study aims to identify α 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 α chains of type IV collagen, α1(IV) through α6(IV) chains. Primordial follicles showed a distinct immunoreactivity against all six α chains in their basement membranes. Immunolabeling for α3(IV) and α4(IV) chains was almost eliminated in the primary follicles. In basement membranes of secondary and Graafian follicles, the immunofluorescent reaction of α3(IV) and α4(IV) chains disappeared in Graafian follicles, a partial reduction in fluorescent immunostaining intensity to α5(IV) and α6(IV) chains was observed; only α1(IV) and α2(IV) chains were not degraded throughout follicular development. On atretic follicles, in addition to α1(IV) and α2(IV) chains, α3(IV), α4(IV), α5(IV) and α6(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 α1(IV), α2(IV), α5(IV) and α6(IV) chains but not α3(IV) and α4(IV) chains. We expect that the decrease in α(IV) chains prompts follicular development and is a prerequisite condition for follicular maturation.