The fate of injected S(35)-labeled sulfanilic acid-azoalbumin in the serum, various organs, and liver fractions was compared in the newborn and adult rabbit and in specifically unresponsive and normal adult rabbits. Exponential decay of the injected antigen without the usual immune phase of elimination was observed in newborn and unresponsive adult animals. Comparison of organ distribution of radioactivity in adults and animals injected at birth and 21 days of age showed persistence in the liver at least as long as 3 weeks in all groups (which was the time chosen for observation). The slight differences in spleen and thymus concentrations with age are of undetermined significance. Comparison of the organ distribution of antigen in unresponsive adult rabbits and in normal ones showed slight differences which were similar to those predicted from previous immunization of adults which gave low grade antibody response. There was a slight selective accumulation of antigen in the nuclear fraction of liver homogenates of unresponsive animals, but no other differences were observed.