The alkaline comet assay was used to further characterize the induction of DNA-protein crosslinks (DPX) by formaldehyde (FA) and their removal in the human lung cell line A549 and in primary human nasal epithelial cells (HNEC). DPX were indirectly measured as the reduction of gamma ray-induced DNA migration. FA induced DPX in A549 cells in a concentration-related manner in the range of 100-300 microM. This result is in agreement with previous studies using different mammalian cell lines. The main new findings of the present study are: (i) Determination of cytotoxicity in relation to genotoxicity strongly depend on the method used. Cytotoxicity measured as the reduction in cell counts 48 hr after addition of FA to the cultures occurred parallel to the induction of DPX while colony forming ability was already reduced at 10 times lower FA concentrations; (ii) DPX induced by a 1-hr FA treatment were completely removed within 8 hr cultivation in fresh medium while in the presence of FA in the medium DPX levels remained unchanged for 24 hr; (iii) Induction and removal of DPX did not fundamentally differ between exponentially growing and confluent A549 cultures; (iv) Slowly proliferating HNEC showed the same sensitivity towards FA-induced DPX as A549 cells (i.e. the same FA concentrations induced DPX under the same experimental conditions) and removed DPX with a similar efficiency. In summary, these results contribute to a better understanding of the genotoxic activity of FA in vitro and indicate that the tested cultured primary and permanent human cells do not differ fundamentally with regard to the processing of FA-induced primary genotoxic effects.