Two studies were conducted on endurance horses predominantly of Arabian breeding participating in an 80 km ride dedicated to research in April 2001 (Trial 1) and April 2002 (Trial 2). Objectives were to determine effects of endurance exercise, antioxidant supplementation, and a feed rich in fiber and fat vs. a high fat sweet feed on immunoglobulin A and G concentrations as well as identify selective IgA deficiency in endurance horses of Arabian breeding. There were no effects of distance in Trial 1 on IgA (P = 0.73) or IgG (P = 0.18) concentrations. In Trial 2, IgA concentrations increased (P = 0.05) and IgG concentrations increased (P = 0.006) after the start of the race. There were no effects of antioxidant supplementation on IgA (P = 0.16), IgG (P = 0.16), and IgM (P = 0.70) concentrations. There were no diet effects on IgA (P = 0.80), IgG (P = 0.59), and IgM (P = 0.54) concentrations. There were horses in both trials that were deficient in IgA only. Concentrations of IgG and IgM were within normal ranges, and there were no differences in training, performance and transportation variables, IgG concentrations, antioxidant supplementation, and feed supplementation compared to the horses with normal IgA concentrations. The concentration of IgM was higher in IgA deficient horses in Trial 1 (P = 0.035) and Trial 2 (P = 0.017). Horses with deficient IgA tended to be associated with health problems commonly found in humans and dogs affected with selective IgA deficiency.