The occurrence of a chronic seronegative polyarthritis has been studied in four families in which the proband presented with some form of juvenile rheumatoid arthritis. In these families, histocompatibility testing suggested that susceptibility to arthritis was controlled by a dominant allele with variable penetrance and expressivity at the rheumatoid-like arthritis, first locus (RLA-1). The combined lod scores for the four families (2.70) indicated that the odds in favor of genetic linkage between the major histocompatibility complex and the postulated disease susceptibility gene, RLA-1, were 500:1. In one family, a recombinant event permitted localization of RLA-1 centromeric to HLA-D. Of major interest was the fact that there was significant pleomorphism in the clinical manifestations of arthritis in affected individuals. In some, symptoms first occurred in childhood and in others, in adult life. Even among those with childhood-onset arthritis, different types of juvenile rheumatoid arthritis were observed within the same family.