We have investigated the effect of treatment with epidermal growth factor (EGF) and/or monoclonal antibodies (Mabs) to the epidermal growth factor receptor (EGFR) on the growth in vitro of a number of human tumour cell lines. Mabs ICR10, ICR11, and ICR16 that prevent the binding of both I-125-EGF and I-125-TGFalpha to the EGF receptor were found to inhibit the growth of human tumour cell lines overexpressing the EGF receptor. At high concentrations (5 nM), EGF was also found. to inhibit the growth of HN5, A431 and MDA MB-468 cells whereas the proliferation of SKBR3 and HN6 cells was stimulated. While some of the cell lines (e.g. HN5 and HN6) were very susceptible to growth inhibition by antibodies to the EGFR others, known to secrete autocrine growth factors (e.g. A431 and MDA MB-468), were less affected by these antibodies. Indeed, growth of the latter cell lines was inhibited more effectively by the addition of 5 nM of exogenous EGF than by treatment with 156 nM of the anti-EGFR Mabs ICR16, ICR11, and ICR10. In addition, we show that the growth of HN5 cells, which express very large numbers of EGF receptors (1.4x10(7)/cell), was stimulated at picomolar concentrations of EGF but inhibited at concentrations of EGF in the nanomolar range. Maximal stimulation (25% above control) was observed with the addition of 78 pM EGF to the cultures. The mitogenic effect of low concentrations of EGF and the growth inhibitory effect of nanomolar concentrations of EGF on HN5 cells could be reversed by the addition of 156 nM of the anti-EGFR antibodies. We conclude that growth inhibition can follow from either inhibition of signal transduction by blocking ligand binding to the EGFR or from excess binding of ligand. With tumours that produce significant quantities of autocrine growth factors the latter will compete with the Mabs for binding to the receptor and reduce their effectiveness.