We reported previously that accumulation of myelin basic protein (MBP) in foetal brain aggregate cultures is enhanced by supplementation with peritoneal macrophages. The present study demonstrates that the rate of MBP accumulation in macrophage-enriched cultures continues to increase over time unaccompanied by a matching increase in the oligodendrocyte marker cyclic nucleotide phosphodiesterase, while that of control cultures reaches a plateau. These observations are supported by electron microscopic evidence of cumulative numbers of myelinated axons in the aggregates over time and by enhanced expression of myelin protein genes in macrophage-enriched relative to control cultures. Aggregates demyelinate following short-term exposure to cytokines and anti-myelin oligodendrocyte glycoprotein antibody, and MBP synthesis resumes following removal of demyelinating agents. Supplementation of cultures with macrophages influences the degree of myelin breakdown and remyelination, drawing attention to the role that macrophage-derived growth factors may play in myelinogenesis and myelin repair in inflammatory demyelinating disease. J. Neurosci. Res. 47: 384–392, 1997. © 1997 Wiley-Liss, Inc.