A cell surface antigen present on cultured cerebellar neurones appears to be transiently expressed during cerebellar development in the rat.

A monoclonal antibody has been produced from a fusion of NSO myeloma cells and splenocytes from a mouse immunized with cultures from early postnatal rat cerebellum. The binding of this antibody designated 69A1 is concentrated in the molecular layer of the developing rat cerebellum during the first two weeks postnatally but falls below the level of detection during the third week. Immunoelectron microscopy has shown antibody binding in the molecular layer to be confined to the parallel fibres of the granule neurones. The disappearance of binding coincides with a period during which the formation of new parallel fibres is completed and rapid synaptogenesis within the molecular layer begins.