Mounting an immune response has been suggested to be physiologically costly because of metabolic requirements of immune cells specifically and upregulation of the immune system in general. We investigated such costs in free-living water pythons (Liasis fuscus), immunized with a harmless antigen, keyhole limpet hemocyanin. In the present study, we analyze the independent effects of age, blood parasite load, and body condition on the ability to mount a humoral immune response (level of antibody production to novel antigens). Python humoral immune response decreased with increasing body length/age, decreased with increasing blood parasite load, and decreased with declining body condition. The results suggest an energetic trade-off between immunocompetence and other energetically costly processes. Key words: age, condition, humoral immune response, parasites, python.