Localization of p53 in human cultured lymphocytes and in cultured skin fibroblasts was studied by immuno-fluorescent microscopy and post-embedded immunoelectron microscopy using Lowicryl K4M. In quiescent lymphocytes, p53 was found in small amounts in both the cytoplasm and the nucleus. p53 in the nucleus was found associated with the non-chromatin structure. At 24 h or 72 h of PHA stimulation, p53 increased markedly just beneath the plasma membrane and in the nucleus, which stained diffusely with anti-p53. In resting fibroblasts, small amounts of p53 were present in both the cytoplasm and the nucleus. After 16 h of stimulation of confluent-resting fibroblasts by trypsinization and replating, a phase just prior to the initiation of DNA synthesis, p53 slightly increased in both the cytoplasm and the nucleus. Afterwards, p53 was present predominantly in the cytoplasm, closely associated with the cytoskeletal actin filaments. In mitotic cells, p53 was distributed throughout the cytoplasm. When fibroblasts were extracted with saponin, p53 was still associated with the actin filaments, as well as mitochondrial membranes and granular structures of the nuclear matrix. Our data suggest that the initial increase of p53 in cells that enter the cell cycle through G1 first bind to the actin cytoskeleton, and that some of the p53 then move into the nucleus to initiate gene activation and DNA synthesis for cell proliferation. This implies that there is some functionally significant interaction between p53 and actin in the cells.