The morphology and behavior of hepatocellular neoplasms in control B6C3F1 mice were studied and classified according to cell size, tinctorial properties of the cytoplasm, pattern of growth, and other morphological features. The small tumors were composed primarily of basophilic hepatocytes which grew in a solid adenomatous pattern. Larger solid tumors had foci of prominent trabecular formations. All mice with metastatic hepatocellular neoplasms had large areas of prominent trabecular formations in the primary liver tumors. It appears that trabecular hepatocellular carcinomas develop from adenomatous neoplasms in the liver of B6C3F1 mice. The histogenesis and morphology of hepatocellular neoplasms in B6C3F1 mice is compared with those induced by carcinogens and differences are noted.