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Immature plasma cell neoplasias induced in BALB/c mice with subcellular leukaemic extracts were investigated with the electron microscope for virus-like particles. A-type particles located in the cisternae of the endoplasmic reticulum and in the perinuclear space were the only virus-like particles observed. The A-type particles average 70 mü in transverse diameter. They display two electron-dense shells around a 30 mü wide electron-translucent core. In most instances the particle surface is covered by a layer of granular material with moderate electron-density. A few electron-dense granules are noted in the core of some A-type particles. The A-type particles apparently are formed by inward budding from the membrane of the endoplasmic reticulum and occur whether or not the neoplasm produces paraprotein. The findings are discussed in light of our knowledge on leukaemic lesions in mice with plasma cell neoplasia.