The transformed phenotype of rat 208F cells transfected with the T24 H-ras1 oncogene is suppressed by simultaneous or subsequent transfection with the normal H-ras1 gene. The suppressed cells express both the normal and mutant forms of ras p21 but the normal form predominates. Rare transformed cells obtained after simultaneous transfection express mainly the T24 p21. Some suppressed cells induce tumours in nude mice after a long lag period and these tumour cell lines have much reduced expression of normal p21. The normal H-ras1 gene also suppresses the transformed phenotype induced by mutant N-ras, albeit less effectively. The tumorigenicity of the EJ bladder carcinoma cell line, which contains only the T24 mutant allele of H-ras1, is also suppressed following transfection with the normal H-ras1 gene. The results suggest that transforming alleles of ras genes do not behave in a fully dominant manner and that expression of the normal allele at elevated levels can lead to suppression of the transformed and tumorigenic phenotypes.