HLA typing studies were performed on a group of 16 patients with acute Brucella-induced reactive arthritis and results were compared to those obtained in 16 patients with acute brucellosis but without arthritis and to 55 normal control subjects. No increased prevalence of any HLA antigen was noted between patients and control groups. HLA-B27 frequency was diminished in the Peruvian population studied. The most interesting finding was the presence of a statistically significant low frequency of HLA-A2 in the disease group suggesting that this antigen may play a protective role for the development of brucellosis.