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The relationship between the type of melanin-based plumage coloration and the strength of experimentally induced immune response was studied using as an example a pied flycatcher population from the Moscow Region. The plumage of pied flycatcher males exhibits the full spectrum of transitions from contrasting black-and-white to cryptic brownish, the latter being very similar to the coloration of females. In spite of numerous studies, the nature of this polymorphism still remains vague. Unlike many other avian species with monocyclic breeding, a considerable fraction of pied flycatchers combines two energy-consuming productive processes, breeding and molt. During the main experimental treatment we activated the humoral immunity of free-living males in chick-rearing period by injection of nonpathogenic multigenic antigen (sheep red blood cells, SRBC) and estimated the strength of the immune responses after repeated captures in 6-8 days. In addition, after each capture we estimated the numbers of leucocytes (WBC), heterophil to lymphocyte ratios (H/L) and measured night time basal metabolic rates (BMR). Non-molting males of different color types showed the same immune responses. Among molting birds, the strength of the immune response was significantly higher in pale males (morphs 4-7 by Drost's scale) than in bright males with rich melanin-based coloration (morphs 2-3). This difference resulted from two opposite processes. During molting, pale males heightened the antibody titer after immunization, while bright males tended to reduce the strength of immune response. Possibly such an asymmetry in immunocompetence at the first stage of molt reflects the different life strategies of pied flycatcher males - conspicuous birds less commonly combine breeding with molt than cryptic ones.