To obtain information about the genetic basis of the HL-A system, 299 unrelated Norwegians were examined with HL-A antisera. The results indicated that the HL-A antigens are determined by multiple mutually exclusive genes at two sub-loci, the LA sub-locus and the 4 sub-locus. Four alleles could be identified at the LA sub-locus: LAI, LA2, LA3 and LA-BS. Additional alleles exist at this sub-locus, with a frequency of 0.2572 in this material. Five alleles were found at the 4 sub-locus: MH, TT, HN, 7C and 7D. Additional alleles also exist at this sub-locus; the frequency of this ‘null’ allele was calculated to be 0.4565.