The transplantable Swarm rat chondrosarcoma grew to twice the weight in 5 weeks in Lewis strain rats (approximately 80 g) as it did in Wistar strain rats (approximately 40 g). Wistar tumor passaged into Lewis rats adopted the accelerated growth rate of the Lewis tumor on the second passage. Conversely Lewis tumor passaged into Wistar rats grew like Wistar lineage chondrosarcoma after two passages. Lewis and Wistar tumors had a similar histological appearance. The extracellular matrix composition of the two tumors was very similar. Tumor explant cultures synthesized about the same amount of 35S-proteoglycans and the same proportion of 3H-hyaluronate: 3H-chondroitin sulfate. Serum levels of growth hormone and insulin were the same in the two strains but T3 and T4 levels were 50% higher in Lewis rats compared to Wistar rats. It is likely that accelerated tumor growth in Lewis strain rats is related to the higher thyroid hormone levels.