Since 1985, fish pathologists at Cornell University have studied lesions in freshwater fish inhabiting natural waters throughout New York State in order to clarify possible adverse effects of contaminants on fish health. In the course of these studies, we have conducted complete necropsies on over 400 brown bullheads and histologic examinations of the major organs of over 370 of these fish. In the course of our histologic studies, we observed islands of well-differentiated hepatocytes in the spleens of neoplasm-free brown bullheads from 2 of the 37 diverse lakes, rivers, and ponds that we sampled. The splenic hepatocyte island occurred in 11-100% of fish from the 2 affected sites, with 1-50 hepatocyte clusters per section of spleen. Hepatocyte islands measured up to 1 mm in diameter. Unlike the metastases of hepatocellular carcinomas, which we have described elsewhere in this issue (9), these islands of hepatocytes in the spleens of neoplasm-free fish were not evident grossly. These hepatocyte clusters in the spleen of certain populations of brown bullheads may lead to confusion in studies of neoplasia in brown bullheads. Caution is advised in interpretation of metastasis of liver neoplasms to the spleen of brown bullheads.