SUMMARYTbe allotypic forms ofthe C3b/C4b receptor (CRl, CD35) differ in length, in the number of expressed C3b binding sites and thus in their ability to mediate the processing of circulating C3-and C4-bearing immune complexes. We have used a combination of three informative restriction fragment length polymorphisms (RFLPs) to assess the frequencies of the F (most frequent allele comprised of four long homologous repeats (LHR)), S (five LHR) and F' (three LHR) alleles ofthe C3b/C4b receptor (CRl, CD35) in a French population of patients with systemic lupus erythematosus (SLE) (« = 63) and healthy controls {n= 158). A significantly higher frequency of tbe S phenotype was observed among patients (51 %) as compared with controls (26%). The F' allele was found in 2/61 patients and 1/85 healthy controls, indicating the rare occurrence ofthe short CRl allele in SLE. This allele is also extremely rare in the normal population. The overrepresentation ofthe S long allele among patients is indicative of a genetic linkage between CRl and susceptibility to SLE.