The erythrocytes of a normal man were agglutinated more weakly than normal cells by several anti-P(1)OO^k sera, and exhibited a decreased capacity to absorb these antibodies. Analysis of his erythrocyte glycosphingolipids revealed that the globoside (P antigen) content was less than 25% of normal, and trihexosyl ceramide (the Pk antigen) was 30-40% of normal. The ganglioside content of his erythrocytes was approximately four times normal and sialosylparagloboside was increased about sixfold. It appears that his erythrocytes are unable to synthesize normal quantities of trihexosyl ceramide, and that these serological and chemical features constitute a new phenotype in the P blood group system.