The author studied the physicochemical properties of phosphorylated proteins by POCI(3) and also studied the reactions of phosphorylated proteins with antibodies of original proteins. And it was proved that the phosphorylated egg albumin (PEa) was different from the original egg albumin (Ea) in the physicochemical properties and also proved that the reactions of PEa with antibodies of Ea were different from the reactions of Ea. And the results were as follows. 1) Comparing with Ea, PEa contained phosphoryl groups. And the quantity of phosphoryl groups increased, in some extent, in accordance with the quantity of POCI(3) additioned in Ea. 2) Comparing with the Ea solution, the PEa solution showed incoagulability on boiling also showing insolubility near the isotonic point and was far more viscous and then showed the slightly fast moving parties and a moving tendency as one component by filter-paper electrophoretic method. 3) The antigen titer of anti-Ea sera decreased in inverse proportion to increasing phosphoryl groups introduced into Ea. 4) The PEa anti-Ea reaction was compared with the Ea anti-Ea reaction by applying the serum agar technic and the number of precipitate bands of the former showed a tendency to get into one by increasing phosphoryl groups introduced into Ea and 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 Ea was kept positive, but this Arthus's reaction was weak than that of Ea. 6) It was presumed from the above results that the phosphorylation of Ea antigen destroyed a part of determinant groups of it.