In an attempt to determine if both pleomorphic adenomas and adenoid cystic carcinomas are derived from myoepithelial cells, 23 pleomorphic adenomas, 22 adenoid cystic carcinomas, and 17 normal salivary glands were examined immunohistochemically by using monoclonal antibodies directed against actin (HUC1-1, 1A4), keratin (AE-1, 34 beta E 12), and vimentin (V9). In normal salivary glands, the myoepithelial cells demonstrated a positive reaction to the monoclonal antibodies against actin and only rarely reacted with those against vimentin. No reaction to those against keratin was noted. In pleomorphic adenomas, cells that histologically resembled myoepithelial cells displayed a positive reaction to HUC1-1 in 60.9% and to 1A4 in 65.2%. In adenoid cystic carcinoma, 59.1% of cases demonstrated a positive reaction to both HUC1-1 and 1A4. These results supported the hypothesis that the majority of pleomorphic adenomas and adenoid cystic carcinomas arise from cells of myoepithelial origin.