In order to increase the knowledge on the human Cl genes polymorphism, sequencing of the most variable region (CM) has been carried out in four Tenth International Histocompatiiility Workshop iymphoblastoid cell lines: PLH, LKT3, YAR and DBB. Barn HI 926 bp fragments were amplifled (Belt et ai., 1985) using specific flanking primers (L3 and LA, Braun et al., 1990), by the PCR method. Purified products were cloned in a plasmid vector and recombinants were identified by differential migration in an 1% agarose gel electrophoresis. Double stranded DNA templates were sequenced using the Sanger’s dideoxy chain terminator method, with dye- iabelied dideoxy terminators and specific C4 oligonucieotidesL3, L5 and L6 (see Figure 1). Each sequence was coniirmed by analysis of 3 or 4 clones to avoid artifact aiieles due to the polymerase reaction. Four 883 bp sequences were obtained, which were assigned to the alleles tXX& C&Vi, C4A91 and CIIBNEW. Isotypic assignation was made based on the residues found in positions 1101, 1102, 1105 and 1106: proline, cysteine, leucine and aspartic acid for C4A and leucine, serine, isoleucine and histidine for c4B (Yu et ai., 1986). The obtained sequences have been compared with the already available ones. C4A2 sequence is identical in each polymorphic residue to C4A3, except codon 1054,