The expression of ras, c-myc and c-erbB-2 oncoproteins in 100 human (73 ductal and 27 lobular) breast carcinomas has been examined using an immunohistochemical analysis. The monoclonal antibody Y13 259 has been used for the ras p21, the monoclonal antibody Myc1-9E10 for the c-myc p62 and the polyclonal antibody pAb1 (from Triton Bioscience Inc.) for the c-erbB-2 p185 oncoproteins. The following conclusions can be drawn from the analysis: Of the 100 breast carcinoma cases studied only 14 did not express any of the three oncogenes. The remaining 86 were positive for one or more of the three oncoproteins. Ductal carcinomas expressed oncoproteins in 92% of the cases (67/73), whereas lobular carcinomas expressed them in 70% of the cases (19/27). The most frequently expressed was c-myc p62 in 70% of cases followed by ras p21, 55% and c-erbB-2, 35%. Elevated expression of ras, myc or erbB-2 oncogenes did not correlate with the presence of metastasis in auxiliary lymph nodes, the numbers of infiltrated lymph nodes the grade of the tumor or hormone status. However, there appears to be a correlation between increased ras staining intensity and patient's age, below 50 years.