Active thymidine incorporation was observed in both parental nuclei in heterokaryons derived via polyethylene glycol-mediated fusion of postreplicative “senescent” human diploid fibroblast-like cells and a thymidine kinase-deficient murine cell line (3T3der-4E). Some increase of the 3H-thymidine labeling index was also apparent in unfused 3T3der-4E cells co-cultivated with senescent cells, consistent with metabolic cooperation. While no human metaphases could be detected in control hybrid preparations, hybrid metaphase figures containing essentially the entire human complement were demonstrated in the fused cultures as early as 24 h after fusion; the morphology of the human chromosome (bi-armed) suggested that the senescent human cells were stimulated to reinitiate replicative DNA synthesis rather than rep