The karyotypic characteristics and clonal composition of human normal diploid MRC-5 (46,XY) and tumorigenic SK-UT-1 (45,XX,-13) and SK-UT-IB (46,XX) cell lines were studied. In MRC-5, 45, XX, -6; 45,X; 46, XY, t(7;14) (p11:q24) and 47, XY, M1 clones were identified. Other karyotypic characteristics included 2.4% polyploidy, 4.8% breaks/gaps, 4.1% structural anomalies, 1.4% hyperdiploidy, and 5.4% hypodiploidy. In SK-UT-1, 45, XX, -13, and clones with t(18;22) (q22.2;q11.23) and 46, XX, 4p+ occurred at 56.6%, 39.3%, and 4.1%, respectively. The t(18;22) karyotypes differed by either paired N13, one N13 plus one i(13q), or one i(13q). The significance of single chromosome amplification relating to the outgrowth in one culture by the t(18;22) clone is discussed. Structural aberrations occurred at high frequencies in N3, N6, and N7, and breaks from these chromosomes were clustered at 3p12, 3q21, 6q21, 6q23, and 7q11.2. The highly malignant SK-UT-IB had the stable karyotype and one 46X, f clone only. Clearly, the cell population was basically mosaic, containing more than one co-existing clone plus the polyploidy of each clone.