Quantitative differences, in terms of antibody plaque-forming cells per mg. spleen, were observed in young and old animals of the BALB/c and NZB mouse strains. Ultrastructurally, antibody-producing cells from spleens of immunologically hyperactive young NZB mice appeared relatively similar to those from overtly autoimmune old NZB mice showing depressed immune responsiveness. There was more extensive dilatation of endoplasmic reticulum in cells of older mice, with some of these cells manifesting increased fragility. Viruslike particles, observed to be located intracisternally, lacked an electron dense nucleoid and were not seen budding from plasma membranes; they were thus unlike the murine leukemia type-C particles reported to be found in organs of NZB mice. Additionally, since these viruslike particles were observed in antibody-producing cells of young and old nonautoimmune BALB/c mice as well as in those of the NZB strain, it is probable that they do not represent the agent responsible for development of autoimmune disease or malignancy.