Parasites and disease have been increasingly recognized as agents of ecological and evolutionary importance. Because the endocrine and immune systems are related, it is also likely that hormonally controlled traits, such as secondary sexual characters, may be influenced by responses to disease. Testosterone appears to be associated with a suppressed immune system in many vertebrates, and evidence from captive Red Jungle Fowl suggests that the sexes differ in blood parameters involved in immunity and that comb length is negatively correlated with lymphocyte number in males. Immune system parameters may provide a useful gauge of condition in captive and field—caught animals.