Protection of newborn mice against MHV3 infection requires the transfer of several cell populations originating from adult syngeneic donors: adherent spleen cells, T lymphocytes, and a third population present in the nonadherent spleen cell fraction, in peritoneal exudates, and in bone marrow cells (M cells). M cells were found to be sensitive to short-term incubation at 37 degrees C and to preincubation with anti-bone marrow antiserum, mitomycin C, puromycin, and aggregated Ig, the latter suggesting the presence of Fc receptors. They were resistant to silica particles but were sensitive to irradiation with x-rays as well as with 89Strontium. Nonadherent spleen cells, however, behaved differently from M cells toward x-irradiation since they were radio-resistant, suggesting that M cells are precursors that require further differentiation or division to participate in MHV3 resistance. Effector M cells responsible for MHV3 resistance display, therefore, some similarities with natural killing cells. They might belong to a group of effector cells operative in regulatory processes or anti-tumor surveillance but also may be defense mechanisms against infectious diseases.