Canine venereal tumor cells were grown in monolayer and tumor tissue fragments were maintained in vitro for 56 days in medium-199. Attachment and cell replication were evident within 17 hours and monolayer was obtained by the tenth day. Growth at first comprised mainly round cells with abundance of cytoplasm and vesicular nuclei, and a few spindle-shaped cells. Later cells became more elongated and like fibroblasts. Cell degeneration started by the 17th day and most were degenerated by the 24th day. Necrosis and depletion of cell population were prominent in tumor explants during the first 2 weeks. Distinct cell multiplication was evident by the 21st day and tumor fragments were repopulated with cells resembling the original tumor by the 56th day. Two distinct morphological cell types were seen: small cells with vesicular, round to oval nuclei and acidophilic cytoplasm; and the large cells with large hyperchromatic nuclei and acidophilic cytoplasm.