Water-soluble substances bearing HL-A antigenic determinants were extracted from the spleen, lung, liver, and kidney of 3-, 4-, and 5½ -month fetuses. These substances inhibited alloantisera in a pattern consistent with the phenotype of the antigen donor. There were significant differentials in the HL-A antigens which were solubilized from various tissues. Fetal kidney was the best source for the solubilization of HL-A antigens by sonic energy.