Because quality differences between individuals affect fitness, much research has attempted, with limited success, to relate physiological condition (e.g., body reserves), to differences in life history between individuals. Recently, it has been suggested that immunocompetence may reflect condition, and it thus may mediate variation in individual quality and reproductive performance and, ultimately, fitness. We measured humoral immunocompetence (HIC) by immunizing female tree swallows with a harmless antigen and measured the specific antibody responses in a novel enzyme-linked immunosorbent assay developed for passerine birds. HIC was strongly correlated with egg-laying date, an important determinant of reproductive success in female tree swallows. We also investigated the effect of increased workload on HIC by manipulating female flight costs by clipping flight feathers. Clipped females had lower HIC than nonclipped females. These data suggest that HIC is a measure that may reflect phenotypic quality and also appears to be sensitive to increased workload in female tree swallows.