We examined effects of the anti-epidermal growth factor receptor monoclonal antibody C225 on proliferation, cell cycle phase distribution, apoptosis, and radiosensitivity in squamous cell carcinoma (SCC) cell lines derived from head and neck cancer patients. Exposure to C225 in culture inhibits SCC proliferation in a time-dependent manner, and the degree of growth inhibition, compared to controls, ranges from 20 to 75%. Flow cytometry analysis demonstrates that C225 treatment induces accumulation of cells in G1, which is accompanied by a 2-3-fold decrease in the percentage of cells in S phase. C225 exposure also induces apoptosis in SCC populations, as demonstrated by flow cytometry analysis using dual stainings of merocyanine 540 and Hoechst 33342. Western blot analysis indicates that C225 exposure induces accumulation of hypophosphorylated retinoblastoma protein and increases expression of p27KIP1. An increase in Bax expression and concurrent decrease in Bcl-2 expression are observed when SCC cells are exposed to C225. Examination of C225 effects on radiation response in SCCs demonstrates enhancement in radiosensitivity and amplification of radiation-induced apoptosis. These effects are observed in both single-dose and fractionated radiation experiments. C225 represents a promising growth-inhibitory agent that can influence cellular proliferation, apoptosis, and radiosensitivity in SCCs of the head and neck.