Cell proliferation under nonadhesive conditions was examined in anchorage-dependent mouse fibroblasts (3T3 and 3T6), in epithelial monkey kidney cells (BSC-1) and in four B16 melanoma cell variants that exhibit distinct metastatic properties. Non-adhesive conditions established by suspension culture in methyl cellulose or by plating on poly(2-hydroxyethylmethacrylate)-coated plates cause multinucleation and inhibition of cytokinesis in all cell types. Formation of binucleated cells in suspended 3T3 cells continues for 24 hr until DNA synthesis is completely inhibited. Twenty percent of the cells become binucleated during this time. During a similar period in suspension culture over 60% of a synchronized epithelial cell culture (suspended 8 hr after the initiation of S phase) became binucleated. The melanoma cells that maintain a high level of metabolism while in suspension continued to develop into multinucleated cells. After 3 days in suspension, 50% of the melanoma cells have more than one nucleus. Since cell number during suspension culture remains constant, because of a block in cytokinesis, there is a remarkable increase in cell size in suspended melanoma cells. When replated on tissue culture plates, normal cell division resumes by an early extrusion of the extra nuclei in melanoma cells. The melanoma cell variants, although displaying different metastatic potential in vivo, behave similarly in suspension culture. 3T3 cells require 24 hr of replating before recovering DNA synthesis and initiating extrusion of the extra nucleus. The results suggest a differential sensitivity of cytokinesis and karyokinesis to cell-surface contact with the substrate.