A comparative study was made of the histology and growth characteristics of three different Marek's disease virus-induced transplantable lymphomas. These lymphomas were developed previously in related inbred chicken lines G-B1 and G-B2. The UG1 lymphoma was developed by serial i.m. passage in G-B1 chickens, and the UG2 and UG4 lymphomas were developed similarly in G-B2 chickens. While all three lymphomas grow progressively and cause rapid death in syngeneic hosts, differences in pathogenicity exist. For equivalent passage levels, the mean time to death of syngeneic chickens inoculated with 10(5) lymphoma cells was 10.8, 12.8, and 16.3 days postinoculation for UG1, UG2, and UG4, respectively. Histological features examined at the light microscopic level included tumor necrosis, muscle invasion, mitotic activity, and presence of heterophils (comparable to mammalian neutrophils). The UG2 lymphoma was characterized by a high degree of necrosis during all stages of growth. This feature was least pronounced in UG4 lymphomas, which generally grow to a much larger size than UG1 or UG2 lymphomas. Vascular invasion was a feature of UG2 lymphoma cells in skeletal muscle and may account for the necrosis. The UG2 cells, which are somewhat larger than UG4 cells, occasionally contained cytoplasmic vacuoles. While the number of heterophils was highest in early stages of UG2 tumors, the role of these cells is unclear. The findings provide the basis for utilizing the transplantable lymphomas as a model to study mechanisms underlying variable pathogenicity of malignant tumors.