With the use of a recently proposed, combined immunomorphologic classification (Pattengale-Taylor, 1981) for murine lymphomas and related leukemias, 70 spontaneously occurring lymphoid cell neoplasms from female BALB/cStCrl mice were evaluated and classified and then compared to the earlier Dunn classification (Dunn, 1954). The predominant lesion (Le., 60% total incidence) in female BALB/c mice [previously called “reticulum cell sarcomas (neoplasm)”, type B, by Dunn] was a lymphoid cell neoplasm derived from follicular center cells (Le., follicular center cell [FCC] lymphoma). The B-cell nature was further confirmed in the majority (76%) of these FCC lymphomas by means of immunoperoxidase techniques that demonstrated the presence of cytoplasmic immunoglobulin (Clg). Smaller percentages of B-Iymphoblastic (10% total incidence) and B-immunoblastic (7% total incidence) lymphomas were also observed. In addition, FCC, B-lymphoblastic, and B-immunoblastic lymphomas occurred in female BALB/c mice over 20 months of age. In contrast, Clg-negative lymphoblastic lymphomas involving the anterior mediastinum (and presumably T-cell in origin) occurred with a bimodal incidence in both young (13% total incidence at a mean age <6 mol and old (10% total incidence at a mean age >21 mol female mice.