In order to develop cell substrates suitable for the isolation of human oncornaviruses, a large number of human tumors, mostly sarcomas, were cultured in vitro. All explants yielded primary outgrowth of cells. These were of various nature and morphology and exhibited different growth potentials. About 40% of the tumors yielded cell strains which were undistinguishable from diploid human embryonic cell strains. About 20% yielded ‘difficult’ diploid cultures; 35% yielded short-term cultures mostly composed of undefinable cells, rarely of clearly recognizable tumor cells. About 5% yielded cell strains or lines which were aberrant in morphology or karyotype, or both. The malignant nature of the cell strains obtained in our study is conjecturable. Although some of the cell cultures showed evidence of tumoral origin, none complied with currently employed in vitro criteria for malignancy such as lack of topoinhibition and high density growth.