Antibodies reactive with human epidermal growth factor receptor (EGFr), such as 225 IgG1 (Masui et al., Cancer Res., 44: 1002–1007, 1984), are effective tumor-suppressive agents in xenograft models. In the present study an additional antibody reactive with EGFr was made and compared to 225 IgG1 for antitumor activity as an unmodified antibody or as a drug immunoconjugate. This IgG1 clone, designated EGFrL11, competed with EGF and immunoprecipitated a Mr 178,000 protein identical to that immunoprecipitated with 225 IgG1. Cross-competition and immunodepletion studies indicated that the two antibodies bound to distinct epitopes on the same molecule. Immunofluorescence studies confirmed that the EGFrL11 epitope was expressed on the surface of viable human squamous cell carcinoma lines including T222. Unmodified EGFrL11 and 225 IgG1 were tested for antitumor activity in T222 xenografts. At a dose of 81 mg/kg given twice weekly for 3 weeks, tumor suppression, but not regression, occurred with EGFrL11. A similar result was obtained with 225 IgG1. To gauge the potential of these antibodies as immunoconjugates, both were tested for antitumor activity in the T222 model after conjugation to the Vinca derivative 4-desacetylvinblastine-3-carboxhydrazide. Both immunoconjugates completely regressed established tumors. These data suggest that Vinca conjugates with EGFr-reactive monoclonal antibodies warrant further investigation as possible clinical candidates.