A and -B antigens incidence were investigated in 100 cases of Seronegative Spondyloarthropathics and in 303 control individuals of a Venezuelan Mestizo population. Significative increased incidence of B27 was found among 69% of patients with ankylosing spondylitis and 68% with Reiter syndrome being the incidence in the control group o f 2.9%. The B27 negative patients showed 40.4% positivity for Bw35, with spondylitic lcsion in 12 out of 17 cases. Our results question the applicability of B27 as diagnostic criteria in the Mestizo patients bearers of a Seronegative Spondyloarthropathy and suggest the need to investigate further the role of antigens of the B locus in the pathophysiology of these clinical cntitics.