Demonstration of different myelin proteins (myelin basic protein [MBP], proteolipid protein [PLP] and myelin oligodendrocyte glycoprotein [MOG]) is used as a tool to determine the stage of MS lesions in autopsy tissue. Since such tissue can never be obtained at well-defined stages of lesion formation, the time course of myelin degradation in MS lesions can only be estimated. In order to obtain a more precise indication on the sequence of events of myelin degradation in MS lesions, the breakdown of human myelin by human monocytes was studied in vitro. Human monocytes were fed with myelin; next cytocentrifuge preparations were made on several time points (day 0 until day 6). The cytospots were immunocytochemically stained with mono- and polyclonal antibodies directed against various myelin proteins (MOG, MBP, PLP). We found that MOG is degraded after 1 day, whereas PLP and MBP can be detected for a longer period, 2 and 3 days, respectively. The exact time frame of myelin degradation in our in vitro assay cannot be extrapolated to the MS lesion formation in vivo, but our data allow conclusions on the sequence of events as well as a rough indication of the time frame of myelin degradation by macrophages in MS lesions.