HL-A antigens were determined in Haida and Bella Coola native Indians, two communities known to have a high prevalence of ankylosing spondylitis. Tests were conducted on those with x-ray evidence of sacro-iliitis and on a sample of the community at large. Sacro-iliitis was found to prevail in approximately 10 per cent of adult Haida males and in over two per cent of Bella Coola adult males. Of 20 Haidas with sacro-iliitis, 17 were HL-A 27 positive. Fifty per cent of the Haida community at large were HL-A 27 positive. Three Bella Coolas known to have sacro-iliitis were all HL-A 27 positive, while 25 per cent of the community sampled at large were HL-A 27 positive. About one in five adult Haida males who were HL-A 27 positive showed evidence of sacro-iliitis, a proportion close to that ascertained in Caucasian communities. It would appear, therefore that the risk of disease in HL-A 27 positive Bella Coola males is considerably lower.