Characterization and histopathological correlation of cytosol proteins of benign and malignant breast tumors. Significant differences in cytosol protein level exist between normal/benign and cancerous breast tissues. There is a positive correlation between the cytosol protein level and histological grade of carcinoma. Well-differentiated carcinoma have a lower value of cytosol protein than poorly differentiated carcinoma. In slab gel electrophoregrams, the total numbers of bands are almost identical in normal, benign, and malignant conditions. In addition, 37 Kd protein band is consistently present in malignant cases and always absent in normal or benign cases. More extensive biophysical examination of this band may provide further insight into the protein alterations in cancer cells at the molecular level. 