Thirteen families were studied for the frequency of HLA-B27 and allotypes of the components of complement, C2, C4 and Bf. The 13 probands all suffered from definite ankylosing spondylitis (AS) as defined by the New York criteria. The results showed that HLA-B27 positive haplotypes in AS patients possessed different complement allotypes. Thus, it is postulated that the gene conferring predisposition to the development of AS is either HLA-B27 itself or another gene in very close proximity to the HLA-B locus.