Ten human ribosomal core proteins examined electrophoretically showed significantly less polymorphic variation than expected on the basis of other protein systems studied. This implies that selective forces may restrict variation at these loci. Variation in common multigenic traits must be determined by a combination of common alleles. If there is restriction of variation at certain types of loci, a limit is placed on the number of loci which can play a role in multigenic inheritance. Surveys for polymorphism in groups of proteins with similar structural or functional roles will further define these limits.