Adhesion mutants were selected from a human lymphoblastoid cell line. Initially, cells were selected on the basis of survival in serum-free medium. Subclones that grow as single cells rather than macroscopic aggregates were selected from the serum-independent variant. The defect in cell-cell adhesion is stable over many generations and is not corrected by growth in serum or the presence of serum in the culture medium. Analysis of mixed cultures composed of adhesive cells and nonadhesive cells indicates that the two cell types do not interact to form mixed aggregations. Furthermore, those results suggest that the adhesion-deficient phenotype does not result from the production of a transferable inhibitor. In a previous study [Whipple, A.P., Dalvin, M. & Millis, A.J.T. (1978) Exp. Cell Res. 116, 457-461], we found that the growth rate in serum-containing medium is identical for the two classes of cells. This suggests that cell-cell adhesion is not a critical factor in the growth of these cells.