The incidence of allergy to aldehydes (formaldehyde, glutaraldehyde, glyoxal) was examined in 280 health care workers suffering from skin lesions. Allergy was diagnosed in 64 (22.8%) patients. The majority of them (85.9%) were sensitive only to 1 aldehyde. Formaldehyde caused allergy slightly more frequently (13.9%) than glutaraldehyde (12.4%). Only 5 (1.9%) patients were sensitive to glyoxal. The irritant effect of aldehydes to the rabbit eye and skin was tested by the Draize and OECD methods. Formaldehyde and glutaraldehyde showed stronger irritant effect than glyoxal. The sensitizing activity of aldehydes was also confirmed in guinea pigs (using the Maximization Test and the OECD methods). Formaldehyde showed the strongest and most persistent reactions. Significantly higher eosinophil and basophil counts were found in the blood samples of the sensitized guinea pigs. Cytotoxicity of glutaraldehyde and glyoxal was tested on mouse 3T3-L1 fibroblasts by the Neutral Red Uptake and MTT Reduction Assay. It was shown that both aldehydes were cytotoxic, and that the cytotoxic effect of glutaraldehyde was stronger than that of glyoxal.