Abstract: In male dark-eyed juncos (Junco hyemalis) experimentally elevated testosterone (T) increases attractiveness and frequency of extra-pair fertilizations, but also decreases male parental effort, resulting in no net effect on reproductive success.  Nevertheless, high-T males are rare in the wild, suggesting a selective disadvantage.  One possible explanation for the rarity of high T may be an extended phenotypic effect through which a T-induced decrease in paternal feeding of nestlings results in increased care by females, thereby leaving less energy available for female immune function.  Survival of their social mates is important to males, as it has been shown that males whose mates fail to return the following year are less likely to be mated in subsequent years.  To test whether T-levels of male juncos can influence immune function in their social mates, we measured the acquired immune responses of free-living female juncos using both cell-mediated (wing-web swelling) and humoral (antibody production) tests of immune function.  During the twelve-day period from the hatching of the eggs to the fledging of the nestlings, females were captured, bled, and given two immune challenges.  Measures of mass and condition suggest that mates of control males (C-males) weighed more and were in better condition than mates of testosterone implanted males (T-males).  Because our sample size was small, we could not convincingly assess the relationship between female immune response and social mate treatment with either test of immunity.  When assessing only mates of C-males, however, a significant positive correlation was found between the cell-mediated response and female condition.  We suggest that females forced to invest more energy into maternal effort may devote less energy to their own self maintenance and, to compensate, may forfeit immunocompetence, physical condition, or both.