Tumor host cell hybrids were isolated in vitro from 2 polyoma induced ascites sarcomas and 1 methylcholanthrene induced sarcoma by selection of surface adherent cell colonies. The loosely attached tumor cells were removed by shaking or mild trypsinization. A difference in the sensitivity of the tumor cells and the tumor host cell hybrids to Colcemid treatment provided an additional tool for selection. The hybrids were identified by chromosomal and antigenic markers. Several hybrid lines derived from the fusions between tumor cells and host cells in vivo were tested for their ability to grow progressively in F1 hybrid hosts and were all tumorigenic. The chromosomal constitution of the tumors approximated that of the corresponding line in vitro.