Hybrid cells have been recovered from selective culture medium after fusion of concanavalin-A-activated human lymphocytes with an AKR mouse thymoma (BW 5147). After 6 months of culture twenty-seven out of forty-nine clones still contained human chromosomes. Human chromosome 6 was present in 89% of these clones, and human X in 70%. Clones from one hybrid line contained several human chromosomes. In twelve of the clones carrying human chromosomes, the rosetting with sheep erythrocytes (SRBC) was 3 times as high as in the BW 5147 cell line. All these clones carried the human chromosome 6, and eight clones contained the human X chromosome as well. In some of these clones (25%) chromosome 6 was the only human one present. In the two clones in which human chromosome 6 was completely missing, the rosetting with SRBC was at the level of the BW line. We therefore suggest that genes on human chromosome 6 are responsible for rosetting with SRBC.