Interleukin 2 production was studied in a family with systemic lupus erythematosus (SLE) and a C4Q0 heterozygous inheritance. Autoimmune manifestations seemed to be associated with the HLA haplotype containing the C4Q0 allele, which was shared by all four ill family members. Concentrations of interleukin 2, however, did not associate either with the haplotype or with the clinical or serological manifestations, as diminished concentrations of interleukin 2 were found in only two subjects with SLE. Thus the defect in this family seemed to be acquired rather than genetically conditioned.