A method which allows growth of normal human tissue to be studied in vitro is used to investigate possible interactive effects of radiation and environmentally important carcinogens on oesophageal and urothelial cell growth. Carcinogens chosen were selected for their known or suspected effect on the oesophageal mucosa or urothelium in vivo. The results indicate that with carcinogens alone concentrations can be identified that result in increased proliferation of cells. With radiation alone inhibition of cell proliferation occurs at all dose points examined. However, at precise combinations of radiation and carcinogen, greatly enhanced cell proliferation could be detected, suggesting a synergistic interaction between the two agents. The results may have implications for the design and interpretation of experiments aimed at elucidating early or premalignant changes in epithelial tissues and may indicate hitherto unsuspected interactions between radiation and environmentally important carcinogens.