5-Formyluracil (5-foU) is a major oxidation product of thymine formed in yields comparable to that of 8-oxoguanine in DNA by ionizing radiation. Whereas the mutagenic effects of 8-oxoguanine are well understood, the investigation of the biological implications of 5-foU has so far been limited. Here we demonstrate that 5-formyl-2'-deoxyuridine (5-fodUrd) supplied to the growth medium of Escherichia coli induces several base substitutions at different frequencies at position 461 in the lacZ gene in the following order: A.T-->G.C>G.C-->A.T>G.C-->T.A>>A.T-->T.A>A.T-->C.G. No induction of G.C-->C.G transversions was observed. It is inferred that 5-fodUrd will be incorporated into the DNA during cell growth, forming mispairs with guanine, cytosine and thymine during replication. It, thus, appears that cell growth in the presence of 5-fodUrd may represent a good model for elucidating the cellular effects of 5-foU residues in DNA.