The author studied the serolochemical properties of phosphorylated proteins by POCI(3), that is, the cross-reactions of phosphorylated human serum albumin (PHuSa) with antibodies of phosphorylated crystalline hen's egg albumin (PEa) were observed by applying the precipitin tests (viz. the precipitin ring test and the serum agar precipitin test) and the active Arthus's test given to guinea pigs. And the following results were obtained. 1) PEa, when injected into rabbits, produced antibodies which gave precipitin reaction with PHuSa. However these anti-PEa sera also reacted both with PEa and unchanged egg albumin (Ea). 2) The anti-PEa sera reacted with PHuSa also by the Oudin's serum agar technic and it was presumed from this reaction that the PHuSa anti-PEa reaction was a single antigen antibody reaction system. 3) By increasing phosphoryl groups introduced into Ea, no remarkable changes given to the antigen titer of anti-PEa sera were found. 4) The PEa anti-PEa reaction was compared with the Ea anti-PEa reaction by applying the serum agar technic and the number of precipitate bands of the former was equal with that of the latter. But the moving speed of precipitate bands of the former was slow than that of the latter. 5) The Arthus's reaction between PHuSa and antibodies in guinea pigs actively sensitized to PEa was kypt positive, but as to these animals the Arthus's reactions with both PEa and Ea were also kept positive.