The effect of heat stress on antibody production to sheep red blood cells (SRBC) was investigated during three experiments using chicken lines selected over six generations for high (H) or low (L) plasma-antibody titer to SRBC after primary intramuscular immunization. The chickens were immunized 24 h after a heat-stress treatment (HS) of four periods of 30 min each at a temperature of 42 C with an intervening 30-min period at a temperature of 22 C. For the control treatment (CT), the chicks were handled the same, but at a temperature of 22 C. Antibody titers were measured on 3, 5, 7, 10, and 14 days postimmunization. The intramuscular immunizations, .25 mL of SRBC, were given in all three experiments; an additional intravenous immunization of .5 mL of 14% SRBC was given in Experiment 2 and of .5 mL of 5% SRBC in Experiment 3. A significant effect of the HS treatment on antibody titers (P less than .05 on Days 3, 5, 7, and 10 after immunization) was found only in Experiment 1. The titers decreased in the H line only. The differences between the H and L lines were significant (P less than .001) in all three experiments after both the intramuscular and the intravenous immunizations. Heat stress was found to have little or no effect on antibody production in the lines studied in the present experiments.