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We have examined the expression of factors associated with the growth, differentiation, and chemotaxis of cells of the monocyte/macrophage series in the central nervous system of Lewis rats sensitized to develop experimental allergic encephalomyelitis. CSF-1 mRNA increased significantly over that found in control animals (sensitized with OVA in CFA or CFA alone). The elevation in the levels of this growth factor commenced immediately before the onset of early clinical signs and peaked immediately before maximal clinical incidence of disease. Expression of CSF-1 message declined to base-line values with resolution of the disease process. CSF-1 protein was also detected in the central nervous system at the height of clinical disease. Expression of the receptor for CSF-1, the proto-oncogene c-fms, also paralleled the early disease process. Elevated levels of c-fms mRNA were detected immediately before the onset and peaked at the height of clinical signs of disease. In contrast to CSF-1 levels, elevated c-fms message expression persisted after resolution of the acute phase of experimental allergic encephalomyelitis. Levels of macrophage chemotactic factor-1 message were also elevated immediately before the onset of clinical signs, peaked with the height of clinical disease, and declined with resolution of the disease. Unlike CSF-1 or c-fms, no endogenous macrophage chemotactic factor-1 message was detected in control animals. Macrophage chemotactic factor-1 protein was demonstrated by Western blot in the central nervous system at the height of clinical disease. The results support the conclusion that expression of factors that specifically target cells of the monocyte/macrophage series are an important component of the disease process in experimental allergic encephalomyelitis.