Sixty-three primary lung carcinomas were examined immunohistochemically for the presence of epidermal growth factor receptors (EGF-R). Of these tumors 35 (55.5%) were positive for EGF-R. A positive correlation between overexpression of EGF-R, on the one hand, and high metastatic rate, poor tumor differentiation and high rate of tumor proliferation, on the other hand, was found. From one tumor with an extremely high amount of EGF-R a primary cell culture was established. The addition of anti-EGF-R serum to this culture decreased cell proliferation. Our results support the evidence for a positive relationship between overexpression of EGF-R and invasiveness, poor differentiation and high growth rate of tumor cells. The blockade of EGF-R with specific antibodies suppressed tumor cell proliferation, suggesting a role of EGF-R in tumor progression.