Peripheral blood lymphocytes from 40 children have been examined for genetic variation in their protein constituents by two-dimensional polyacrylamide gel electrophoresis. One hundred six polypeptides chosen without respect to genetic variability were scored in gels from the 40 children. For each child, gels from both parents were also examined to substantiate the genetic basis of variants observed. Of the total of 4,240 polypeptides, 23 could not be scored unambiguously. Fourteen of the polypeptides showed genetic variants in one or more of the children. One hundred twenty-nine of 4,217 polypeptides scored exhibited the combination of a normal and a variant polypeptide. All variants were present in at least one of the parents of the subject. The index of heterozygosity observed (3.05% +/- .23%) indicates substantial genetic variation in cellular protein constituents.