Murine polyomavirus-induced hamster tumors revealed an unexpected heterogeneity with respect to patterns of cytoskeletal proteins expressed in different visceral and subcutaneous tumors and with respect to viral gene expression early during tumor outgrowth. All tumors analyzed expressed vimentin. Desmin was found in all heart tumors, to variable degrees in kidney tumors and in trace amounts only in 1 out of 4 s.c. tumors. The alpha-smooth-muscle actin isoform was observed in heart tumors only and was restricted to structures that we interpret as being proliferating pericytes or proliferating smooth-muscle cells of the media. In kidneys of infected newborn animals and before the appearance of macroscopic tumors, viral early mRNAs were transcribed from free viral genomes. In tumor tissue the size of the viral transcripts was altered, suggesting that they were transcribed from integrated viral DNA. Since in each tumor discrete bands of viral RNAs were detected, individual tumors arose presumably from single cells and one functional integration event. In contrast, in situ hybridizations of kidney tissue and tumors showed large quantitative differences in viral gene expression not only between different tumors but also between individual cells of the same tumor.