Mouse cells exposed to formaldehyde (FA) were examined for colony-forming ability and DNA synthesis as well as the induction of DNA damage. Cell survival was unaffected until the FA concentration exceeded 150 microM after which there was an exponential decay in survival. Formaldehyde-treated cells studied by the DNA-alkaline-elution technique exhibited a low frequency of single-strand breaks but extensive DNA-protein crosslinking. These crosslinks were repaired following removal of the drug. The DNA damage was accompanied by inhibition of DNA synthesis.