The effects of a new epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor, tyrphostin AG 1478, were tested on three related human glioma cell lines: U87MG, which expressed endogenous wild-type (wt) EGFR, and two retrovirally infected U87MG cell populations which over-expressed either wt (U87MG.wtEGFR) or truncated EGFR (U87MG. delta EGFR). Although AG 1478 inhibited cell growth, DNA synthesis, EGFR tyrosine kinase activity, and receptor autophosphorylation of each cell line in a dose-dependent manner, it was significantly more potent in U87MG. delta EGFR cells than in the other two cell lines. The increased inhibitory response of U87MG. delta EGFR cells was due to a greater sensitivity of the constitutively autophosphorylated Mr 140,000 and 155,000 delta EGFR species to AG 1478. These results suggest that AG 1478 is a relatively specific inhibitor of the delta EGFR, and this finding may have important therapeutic implications since the delta EGFR occurs frequently in glioblastomas and in breast, lung, and ovarian cancers.