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 sacroiliitis and on a sample of the community at large. Sacroiliitis was found to prevail in approximately 10% of adult Haida males and in over 2% of Bella Coola adult males. Of 20 Haidas with sacroiliitis. 17 were HL-A 27 positive. Fifty percent of the Haida community at large were HL-A 27 positive. Three Bella Coolas known to have sacroiliitis were all HL-A 27 positive, while 25% 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 sacroiliitis, 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.