Particles resembling murine leukaemia virus in New Zealand black mice.

Particles which morphologically resembled murine leukaemia virus were detected by electron microscopy in the tissues (spleen, thymus, inguinal lymph nodes, bone marrow or pancreas) but not in serum or plasma pellets of untreated conventional New Zealand Black (NZB) mice aged 1–82 weeks. They were also found in the corresponding tissues of NZB mice which had been thymectomized shortly after birth. The presence of similar particles in the spleen, thymus or pancreas of conventional NZB embryos and, additionally, in the lymph nodes of NZB mice which had originally been introduced into a germ-free environment by Caesarian section and fostering on germ-free mice of another strain, suggests that the virus is transmitted `vertically' through the germ cells or placenta. Preliminary investigations showed similar particles in the organs of conventional F (NZB × NZW) hybrid and New Zealand White (NZW) mice. Large numbers of particles also resembling murine leukaemia virus were found in the spleen and in plasma or serum pellets of young conventional NZB mice which had developed reticulum cell neoplasia following serial passage of lymphoid cell suspensions from ageing conventional NZB donors. The possible relationship of these particles to the autoimmune reactions and malignant changes which occur spontaneously in conventional NZB mice is discussed.