We investigated the effects of sequential immunizations with seven different protein antigens (bovine gamma-globulin, keyhole limpet hemocyanin, rabbit gamma-globulin, tetanus toxoid, SRBC, human serum albumin, and BSA) on metabolic parameters (growth, food uptake, total heat production, metabolizability, energy retention as fat, and energy retention as protein) and antibody responses to these antigens in growing layer chickens. Energy and immune parameters of repeatedly immunized birds were compared with control birds in climate-respiration chambers in three trials. The last antigen (BSA) was given to both groups. Energy retention as fat was significantly higher in the repeatedly immunized group (P < 0.05). Antibody titers directed to BSA were similar for the repeatedly immunized groups and the control group. In addition, the separate antibody responses of the repeatedly immunized group to the administered antigens were compared with the antibody response of a control group receiving only a single antigen at the same age. With respect to several antigens, differences between groups were found. In the repeatedly immunized group, higher Day 0 (base line) titers were found to new antigens, introduced especially during the second half of the experiment. In these birds, significantly lower increases of antibody titers to some, but not all, antigens were found as compared to birds that were immunized only with the same antigen. It was concluded that repeated activation of the humoral immune response in growing layer chickens stimulated fat deposition but had little effect on other metabolic parameters. Repeated immunization had little effect on the immune response to separate antigens in the present experiments.