Fc receptor-dependent myelin phagocytosis has been proposed as a mechanism of demyelination in multiple sclerosis (MS). The object of this study was to determine whether MS patients' sera are more opsonic for myelin than normal controls' sera. Opsonization was tested by culturing thioglycollate elicited murine peritoneal macrophages with 125I-labelled, serum-sensitized bovine central myelin. The two groups of sera were found to opsonize myelin equally well after 30 and 120 min incubations. We conclude that MS patients' sera are not more opsonic for myelin than sera from normal controls.