Summary Fifteen-day balance trials were done with 14 healthy infants of about 34 days of age for Sr90, stable strontium, calcium and phosphorus. Average retentions were: 28% Sr90. −71% Sr; 31% Ca; 30% P. The loss of stable strontium from the body as compared to the retention of the other substances most probably is accounted for by the decreased intake of stable strontium that occurred at the start of the observational feeding period. Dietary Sr90 and calcium were added to the bodies of the infants in a ratio that was about 0.9 of the Sr90 to calcium ratio existing in the diet. This corresponds approximately to an ORbody/diet of 0.9 which contrasts with the accepted value of 0.25 for adults and indicates a lesser capability of the very young to discriminate against strontium in favor of calcium.