Proteolipid protein (PLP) is a major structural component of central nervous system (CNS) myelin. Evidence exists that PLP or the related splice variant DM-20 protein may also play a role in early development of oligodendrocytes (OLs), the cells that form CNS myelin. There are several naturally occurring mutations of the PLP gene that have been used to study the roles of PLP both in myelination and in OL differentiation. The PLP mutation in the jimpy (jp) mouse has been extensively characterized. These mutants produce no detectable PLP and exhibit an almost total lack of CNS myelin. Additionally, most OLs in affected animals die prematurely, before producing myelin sheaths. We have studied cultures of jp CNS in order to understand whether OL survival and myelin formation require production of normal PLP. When grown in primary cultures, jp OLs mimic the relatively undifferentiated phenotype of jp OLs in vivo. They produce little myelin basic protein (MBP), never immunostain for PLP, and rarely elaborate myelin-like membranes. We report here that jp OLs grown in medium conditioned by normal astrocytes synthesize MBP and incorporate it into membrane expansions. Some jp OLs grown in this way stain with PLP antibodies, including an antibody to a peptide sequence specific for the mutant jp PLP. This study shows that: (1) an absence of PLP does not necessarily lead to dysmyelination or OL death; (2) OLs are capable of translating at least a portion of the predicted jp PLP; (3) the abnormal PLP made in the cultured jp cells is not toxic to OLs. These results also highlight the importance of environmental factors in controlling OL phenotype.