A comparative study of the products of the cell cycle control genes p53 (mutated form), p21, Rb (nonphosphorylated and phosphorylated form) and TGFbeta was performed by immunohistochemistry and Western blot, in benign breast disorders and breast cancer (in situ and infiltrating tumors). For the five proteins studied, the relative numbers of positively stained cells were higher in in situ carcinoma than in benign breast diseases. In infiltrating breast tumors, the relative numbers of positively stained cells were even higher than in in situ tumors except for the percentage of pRb immunostained cells, which decreased slightly in infiltrative tumors. For the other four proteins, the percentages of positively stained cases were similar to those found in in situ tumors. In the three groups of patients, TGFbeta immunoreaction appeared in the cytoplasm while immunoreactions to p53, p21, Rb, and pRb were found always in the nucleus except for p21 in in situ tumors, which showed cytoplasmic immunoreaction. Present results suggest that accumulation of mutated p53, cytoplasmic p21, and pRb in breast gland epithelium might be a crucial point in the development of in situ adenocarcinoma. In the infiltrating tumors, the expression of p21 in the nuclei and the decrease in pRb expression suggest an insufficient attempt to hinder cell proliferation.