A high-grade malignant lymphoma that arose spontaneously in a C57BL mouse has been transplanted by intraperitoneal injection of a single cell suspension for over 20 years. In the current study it was characterized cytohistologically and immunologically and was found to be a high-grade malignant (immunoblastic) B-lymphoma. The distribution in the various organs and the peripheral blood was investigated after intraperitoneal and intravenous injection and the effect of splenectomy was studied. The findings suggest that the major initial proliferation of lymphoma cells occurs in the spleen both after intraperitoneal and intravenous injection. The second, final stage is characterized by leukemic infiltration throughout the body organs. The current observations are compared to those concerning the previously described, low-grade malignant murine BCL1-lymphoma.