Sternal hyaline cartilages of Syrian hamsters were dissociated with collagenase and culured. Primary monolayer cultures of the dissociated cells were morphologically homogeneous. Secondary cultures of the chondrocytes were treated with 1 x 10(-6) or 2 x 10(-6) M 4-nitroquinoline-1-oxide (4NQO) for 3 hours or with 5 or 10 microgram 3-methylcholanthrene (MCA)/ml for 3 days. Cultured chondrocytes transformed morphologically 29-51 days after 4NQO treatment and 41-61 days after MCA treatment and began to grow continuously in vitro. Cultures of transformed cells, like transformed fibroblasts. Untreated cells and cells treated with dimethyl sulfoxide did not transform within at least 110 days after inoculation. Among the transformed cells, one near-diploid cell line preserved the distinct phenotypic expression of chondrocytes, whereas heteroploid cell lines lost their differentiated features. The near-diploid cell line produced nodules in the cheek pouches of hamsters within a week, by the nodules regressed later. They showed the chondrogenic properties of the original cells and stained metachromatically with toluidine blue. Heteroploid cell lines formed progressive tumors with few chondrogenic features; these tumors were diagnosed as fibrosarcomas.