In order to determine the effect of taurine depletion in primates, two species were selected which differed in their taurine conjugtion of bile acids. Consequently, eight cebus (taurine conjugators) and nine cynomolgus monkeys (glycine conjugators) were raised from birth with soybean infant milk formula lacking taurine. Half the monkeys received a 500 ppm taurine supplement. After 5 months the taurine concentration of plasma, urine and several tissues was greatly reduced in the unsupplemented monkeys. The least depletion occurred in retinal tissue of both species and in bile acids of cebus, whereas cynomolgus monkeys increased the glycine conjugation of their bile acids 125%. Taurine depletion was associated with a significant growth depression (16.8%) in the unsupplemented monkeys, but retinal degeneration was not observed. Neither species demonstrated an appreciable capacity to synthesize taurine as measured by cysteinesulfinic acid decarboxylase activity in liver and brain. The data suggest that dietary taurine is essential for maximum growth, as measured by weight gain, of infant nonhuman primates fed a soy protein milk formula.