The serological and molecular characteristics of the first HLA-B8 variant (B8Jon), located on a haplotype bearing HLA-A1, -B8Jon, -Cw*07, DRB1*0301, DQA1*05, DQB1*0201, BfS, C4AQ0, C4B1 and the microsatellite alleles D6S265-3, 258-11, 105-8, 299-1 and 202-2 are identified and described. This new HLA-B antigen was distinguished from the usual B8 by its lack of reactivity with Bw6 and most Bw4 antisera, together with its failure to react with most antisera cross-reactive with HLA-B8. B8Jon gives a 'weaker' response than HLA-B8 in titration studies with B8 antisera. It cannot be differentiated from the usual B8 by one-dimensional iso-electric focusing. Time course studies, absorption analyses and serological tests on 17 Bw4 antibodies suggest that B8Jon possesses an unusually 'weak' Bw4 epitope. Studies on the Bw4 sequence motif by PCR, using sequence-specific primers, indicate that B8Jon has a Bw4 sequence in common with other HLA-B alleles, including B*4402-B*4405, rather than the Bw6 motif found on the familiar HLA-B8 molecule.