We demonstrate that human fibroblasts of the rare Pk phenotype lack globoside, which was identified as the blood group P antigen, and that p cells possess neither globoside nor trihexosyl ceramide, which was identified as Pk antigen. Our investigations indicate also that these glycosphingolipid patterns are most likely caused by inherited preferential biosynthetic pathways in the abnormal phenotypes rather than by excess catabolism of the antigens. Evidence is presented that the fibroblasts of Pk phenotype lack beta-N-acetylgalactosaminyltransferase (globoside synthetase; UDP-N-acetylgalactosamine:trihexosylceramide beta-N-acetylgalactosaminyltransferase; EC 2.4.1.79) activity, and those of p are deficient in alpha-galactosyltransferase (trihexosylceramide synthetase; UDP galactose:lactosylceramide alpha-galactosyltransferase) and possibly also in globoside synthetase. The diminished globoside synthetase activity in p cells, however, is not caused by the defect in the gene coding for this enzyme. It appears, rather, to be caused by a failure in gene expression because one-third of Pk X p hybrids became able to express P antigenicity with a time lag of 3-4 days after cell fusion [Fellous, M., Gerbal, A., Nobillot, G. & Weils, J. (1977) Vox Sang. 32, 262-268].