When human fibroblast, TIG-1, was growth-stimulated with fetal bovine serum, the induction level of cell cycle-dependent genes was generally much lower in senescent cells than in young counterparts. Exceptionally, the expression level of cyclin D1 in senescent cells was constitutively higher than in young cells and further increased after serum stimulation, which was confirmed by Northern and Western blots and immunoprecipitation. This was also true in other human diploid fibroblast lines, TIG-3 and MRC-5. However, cyclin D1-dependent kinase activity was not detected in senescent cells. When sense- or antisense-cyclin D1 cDNA driven by beta-actin promoter was transfected into young TIG-1 cells, the number of appeared colonies from sense-strand transfected cultures was lower than that from antisense-strand-transfected ones. However, clones expressing cyclin D1 at low or undetectable level which were isolated after transfection with antisense-cyclin D1 proliferated up to the same division limit as untransfected and sense-strand transfected cells. Four clones of SV40-transformed TIG-1 expressed cyclin D1 at moderate levels during their extended proliferative lifespan. It appears that, if the extremely overexpressed cyclin D1 could cause an inhibition of cell proliferation at senescent stage, cellular senescence occurs regardless of overexpression of cyclin D1.