Squamous cell carcinomas of the head and neck (SCCHN) frequently display high levels of the epidermal growth factor receptor (EGFR). Since EGFR is expressed on the cell surface it may form a suitable target for anticancer therapy with anti-receptor monoclonal antibodies (mAb). Besides the interference with receptor/ligand interactions, binding of mAb to EGFR leads to immunoglobulin-coated tumour cells that may induce or enhance non-specific immune effector mechanisms like antibody-dependent cell-mediated cytotoxicity (ADCC). In established cell lines of SCCHN (UM-SCC 11B, 14C, 22B, and 8029 NA) we investigated the antitumour activity of allogeneic peripheral blood mononuclear cells (PBMC) in combination with rat (ICR 62), mouse (EMD 55900), and humanized (EMD 72000) anti-EGFR mAb. In addition, autologous PBMC were available for tumour line UD-SCC 4. The EGFR protein content of the tumour cell lines ranged between 170 fmol/mg protein and 8100 fmol/mg protein, and MCF-7 cells served as receptor-negative controls. PBMC activity against SCCHN targets was determined in 96-well microtitre-plate monolayer cultures by the colorimetric 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay after coincubation for 4 h, 24 h and 72 h at effector target ratios of 1:1, 5:1, 10:1 and 20:1. PBMC subpopulations were obtained by macrophage depletion (plastic adherence) or natural killer (NK) cell enrichment (magnetic bead negative selection). Prolonged time of exposure and increased effector:target ratios revealed marked antitumour activity of PBMC alone. This non-specific immune destruction was enhanced considerably by humanized and rat, but not mouse anti-EGFR mAb. Increased EGFR protein in tumour cells partly correlated with an intensification of ADCC but was accompanied by decreased primary PBMC cytotoxicity. The utilization of PBMC subpopulations suggested a mainly NK-cell-mediated ADCC, which appeared to benefit directly or indirectly, e.g. via the secretion of cytokines, from other PBMC components. In conclusion, humanized (EMD 72000) and rat (ICR 62) anti-EGFR mAb were able to generate strong antitumour ADCC in target monolayers of SCCHN.