The reported associations between HLA antigen DR3 and null (QO) alleles at the C4A and C4B loci and systemic lupus erythematosus are here analysed. The empirical logistic method has been applied to a body of data which included the relevant genotypes. The analysis suggests that the association with null alleles at the C4 loci is primary and the DR3 association is likely to be secondary to this. This may give information as to the location of the HLA-linked disease susceptibility gene.