Freshly passed feces were collected from cats (n = 5-7) given cooked and uncooked commercial canned-type diets, casein and soy protein containing purified diets and a commercial extruded diet. The feces were anaerobically cultured in medium containing either taurine, taurocholic acid or [2-3H]taurine for 24 h at 37 degrees C. Taurine degradation in cultures was greatest for cultures from cats receiving diets reputed to cause taurine depletion. Diaminopimelic acid in feces indicated that differences in taurine degradation rate among groups was associated with differences in bacterial numbers in feces. After 6 h of incubation, < 10% of taurocholate and > 60% of taurine remained. Nearly all the tritium on the labeled taurine was recovered as water. These results indicate that deconjugation and deamination are the initial steps in microbial catabolism of taurocholic acid and that enteric microbial growth may be a major determinant of dietary taurine requirement of cats.