Effects of binding heterocyclic amines to cells of lactic acid bacteria on theirs absorption were investigated. Cells of Lactobacillus delbrueckii subsp. bulgaricus 2038 and Streptococcus thermophilus 1131 bind both 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx). The binding of strain 1131 cells to Trp-P-1 was maximum in the pHs from 4 to 8, but strain 2038 cells bound Trp-P-1 and MeIQx only slightly at pH 7. We investigated the absorption of heterocyclic amines by the small intestine of F344 rats in the presence of these bacterial cells, using an in situ loop technique. The absorption of Trp-P-1 by the small intestine was significantly lower in the presence of strain 1131 cells than in the absence of the cells, but the presence of strain 2038 cells had no effect on Trp-P-1 absorption. Perhaps strain 1131 cells bind to Trp-P-1 at the same pH as that of the small intestine (pH 6-7) and thus decrease its absorption.