The effect of Lactobacillus species on urinary indican excretion in two types of gnotobiotic rats (GBH-9 and GB-5) and in man was studied. L. salivarius, L. plantarum and L. casei colonized in the cecum and colon of both types of rats at levels of 10(7) to 10(8) per gram of contents following one-dose oral administration and caused a significant reduction in urinary indican excretion, whereas two strains of L. acidophilus which colonized at low levels (10(4) to 10(5)/g) did not reduce indican excretion. Daily feeding of Lactobacillus concentrates caused a further significant reduction in indican excretion which was corrected by tryptophan intake, even in the case of L. acidophilus. There was a clear relationship between fecal lactobacillus counts and urinary indican. A significant reduction of indican secretion was obtained only when the fecal counts of lactobacilli exceeded 10(7).4/g in rats. L. casei also had an effect in man, reducing urinary excretion of indican and p-cresol. Reduction in indican excretion seems to be accompanied by decreasing fecal tryptophanase activity in rats and man. A negative correlation was also obtained between fecal resident lactobacilli and urinary indican in man (r = -0.532, n = 28, p less than 0.01). Subjects with lactobacilli at a level of 10(6)/g or more excreted less indican than those harboring the bacteria at levels below 10(5)/g.