EGF, in pharmacological concentrations, inhibits cell proliferation of the MDA-468 human breast cancer cell line. Previously, we have demonstrated that this was characterized by a reversible cell cycle arrest at the G1-S boundary, concomitant with downregulation of mRNA levels for p53 (a point mutant, p53(273.His)). Since p53(273.His) is regarded as a gain-of-function mutant and acts to enhance cell proliferation, we hypothesized that the G1 arrest induced by EGF might be mediated by p53(273.His). In this study, we report an EGF-dependent altered conformation as indicated by immunofluorescence, while no significant immediate effects of EGF-treatment on p53(273.His) protein levels and synthesis were observed. These experiments demonstrated a decreased PAb 240 (mutant-specific) reactivity of nuclear p53(273.His) in EGF-treated cells, while that of PAb 1620 (wild-type specific) was enhanced. Staining with PAb 1801 (pan specific), on the other hand, showed little change upon EGF treatment. Further studies indicated a decreased phosphorylation of nuclear p53(273.His) in EGF-treated cells. These EGF-dependent events were detected early enough to be attributed as causative of cell cycle arrest. We suggest that EGF-mediated, phosphorylation-dependent conformational change in nuclear p53(273.His), and in turn altered p53 function, may be responsible for EGF-dependent growth inhibition MDA-468 cells.