We have used a panel of monoclonal antibodies directed against the low, middle and high molecular weight subunits of neurofilament triplet, to study their expression in mouse cerebellar granule cells. We demonstrate that in situ such cells only express the 2 lower molecular weight subunits either at various developmental stages or in the adult. The same results were obtained in vitro. This pattern of neurofilament protein expression in adult granule cells is therefore similar to that observed in developing neurons but differs from most neurons in the adult brain. The retention of such 'immature' pattern of neurofilament protein expression throughout adulthood could explain the lack of cytologically identifiable intermediate filaments in these neurons when examined with conventional electron microscopic techniques. It furthermore suggests that various neuronal populations might be characterized by the expression of specific subsets of neuronal intermediate filaments.