Hematopoietic neoplasms in the rodent may be classified into lymphoid or nonlymphoid neoplasms. Lymphoid neoplasms include the following morphologic types: follicular center cell, lymphoblast (lymphocytic), immunoblast, plasma cell, and large granular lymphocyte (LGL). Nonlymphoid hematopoietic neoplasms include histiocytic sarcoma, granulocytic leukemia, erythroid leukemia, and mast cell tumors. Most types of hematopoietic neoplasms, exclusive of LGL lymphoma (leukemia), are more common in mice than in rats. Specific strains of mice have a hematopoietic tumor incidence of more than 50% in aged animals. Some strains of rats (i.e., Fischer-344) may have an incidence of over 50% of LGL lymphoma in aged animals. The tumor type and incidence are characteristic for each rat or mouse strain. Hematopoietic neoplasms have been better characterized immunomorphologically in mice than in rats. The specific cell type and tissue of origin for hematopoietic neoplasms may be important for safety evaluation of chemicals. Specific chemicals may induce specific types of these tumors, which may be the same or different from the spontaneous types. Lymphoid cell neoplasms should not be grouped with nonlymphoid neoplasms in determining the toxicity and carcinogenicity of test substances.