We investigated whether in cultures of mechanically dissociated brain cells from newborn mice the reduction of the number of oligodendrocytes influences their proliferation rate. 14-day-old cultures were subjected to complement-dependent anti-galactocerebroside (GC) antibody-mediated cytotoxicity. The cytotoxic treatment completely destroyed oligodendrocytes. Thereafter, GC+ oligodendrocytes progressively reappeared. Their number was 20 and 66% compared to controls, 3 and 7 days after cytotoxicity, respectively. Proliferating oligodendrocytes were detected 3 and 7 days after cytotoxicity by combining the immunostaining for GC with 3H-thymidine autoradiography. The proliferation rate of oligodendrocytes in treated cultures was increased by 100 and 76% compared to controls, 3 and 7 days after cytotoxicity, respectively. These data suggest that the proliferation rate of oligodendrocytes can be influenced by extrinsic factors.