In previous studies of the expression and organization of proto-oncogenes in human breast a significant correlation has been found between amplification of c-myc and c-erbB-2 genes in carcinomas and poor short-term prognosis. Gene expression was estimated by analysis of total RNA from tissues, and similarly assessment of gene organization relied upon extraction of DNA from tissues. The present study has compared the expression of c-myc and c-erbB-2 mRNA as determined by in situ hybridization, and c-myc and c-erbB-2 protein expression detected by immunohistochemistry in a group of carcinomas for which there was knowledge of genomic organization and/or expression. Formalin-fixed, paraffin-embedded tissues of 38 carcinomas were assessed for the presence of c-myc protein, and 13 of these were examined for c-myc mRNA by in situ hybridization. Similarly processed tissue from 14 tumours was tested for c-erbB-2 protein using the antiserum 21N and ten of these carcinomas studied for c-erbB-2 mRNA localization. There was a good correlation between gene amplification, the presence of c-erbB-2 protein and mRNA: both the latter were detected in six of the seven carcinomas with an amplification but in none without. For some carcinomas there was a good correlation between c-myc protein and mRNA levels. Three carcinomas with gene amplification had a lower percentage of cells with detectable protein than showed hybridization for mRNA. Other carcinomas had a lower level of mRNA expression than protein. Neither approach could predict which carcinomas had amplification of the c-myc gene.(ABSTRACT TRUNCATED AT 250 WORDS)