Compounds which can occur as disinfection by-products (DBP's) in swimming pool water were examined for their mucous membrane irritating potential. Previous studies using the rabbit eye test (Draizé test) have shown that the irritating potential of typical concentrations of free and combined chlorine are insufficient to explain the degree of eye irritation that can result from exposure to swimming pool water. Other DBP's which may be responsible for eye irritation include halogenated carboxyl compounds (HCC's) which act as precursors during the formation of chloroform. In this study, a modified HET-CAM Test (Hens Egg Test at the Chorion Allantois Membrane) has been used to investigate the mucous membrane irritating effects of HCC's. Some of the compounds tested were found to have a significantly increased irritating effect when compared to a chlorine/chloramine mixture of the same concentration, several mixtures of HCC's where even more active at lower concentrations than single compounds. However, the irritating effects of individual compounds as well as of mixtures of HCC's were not sufficiently intense to allow the identification of those compounds specifically responsible for the overall observed increase in irritation. HCC's were therefore tested in the presence of aqueous chlorine solution. When combined with aqueous chlorine, a number of compounds exhibited significantly enhanced effects. Our results show that the eye irritating effects of low concentrations of DBP's can be investigated using a modified HET-CAM assay. Moreover, results obtained using this assay suggest that the mucous membrane irritating potential of swimming pool water is a consequence of the effects and synergistic action of a number of DBP's in the presence of chlorine. Further work should be carried out in order to establish an indicator for eye irritating effects of swimming pool water.