Lactic acid bacteria have been reported to have antimutagenic/anticarcinogenic properties in vitro and in vivo. One possible mechanism for this effect involves a physical binding of the mutagenic compounds to the bacteria. The purpose of the present investigation was to study the binding capacity of eight human intestinal or lactic acid bacterial strains for mutagenic heterocyclic amines formed during cooking of protein-rich food. Binding of the mutagens Trp-P-2, PhIP, IQ and MeIQx by the bacterial strains was analyzed by HPLC. There were only minor differences in the binding capacities of the tested strains but the mutagenic compounds were bound with markedly different efficiencies. Trp-P-2 was almost completely bound and the binding tended not to be of a reversible nature. The binding of PhIP, which reached about 50%, was important as PhIP is a major mutagen in the western diet. IQ and MeIQx were slightly less well bound. pH appeared to be of importance for the binding efficacy. Binding correlated well with the reduction in mutagenicity observed after exposure of the heterocyclic amines to the bacterial strains. The results indicate that cooked food mutagenic compounds, commonly found in the western meat-rich diet, can be bound to bacteria from the normal intestinal microflora in vitro.