Myelination was inhibited in cultures of newborn rat cerebellum by exposure to antisera prepared by injecting rabbits with whole guinea pig spinal cord or cerebrosixde mixed with bovine serum albumin. At 15 days in vitro (DIV), when 90-100% of our control cultures were myelinated, antiserum inhibited cultures were washed and refed routine culture medium or medium containing 5-bromo-2'-deoxyuridine (BUdR) or cytosine-B-D-arabinofuranoside hydrochloride (ara-C) in concentrations previously shown to inhibit myelination in our system. The disinhibited cultures myelinated within 2-5 days of removal of antiserum regardless of the presence of BUdR or ara-C. The evidence suggests that the myelin-forming oligodendrocytes do not undergo cell division before myelination during disinhibition from antiserum. In addition, cultures exposed to BUdR on 5-7 DIV in the presence of antiserum and disinhibited at 10 DIV did not have myelinated axons as observed by light microscopy at 15 DIV. Thus, the BUdR sensitive oligodendrocyte division remained intact in antiserum inhibited cultures. We conclude that the antisera inhibit myelin formation without interrupting the pattern of cell division or the covert differentiation of oligodendrocytes.