Social cues may interact with photoperiod to regulate seasonal adaptations in photoperiod-responsive rodents. Specifically, photoperiod-induced adjustments (e.g. reproduction and immune function) may differ among individuals in heterosexual pairs, same-sex pairs, or isolation. Heterosexual cues may be more influential based on their potential fitness value than same-sex cues or no social cues. The present study examined the effects of pair (with a male or female) or individual-housing on reproductive and immune responses in male white-footed mice (Peromyscus leucopus) maintained in long or short day lengths. Female pairing did not affect reproductive responses in short-day males. In long days, however, the presence of a female increased both testosterone concentrations and testes mass as compared to individually-housed and male-paired mice, respectively. Short-day, individually-housed males enhanced delayed-type hypersensitivity (DTH) responses as compared to single-housed mice in long days, but all paired groups decreased DTH responses regardless of photoperiod. The lack of enhanced DTH response in male mice paired with females coincided with reduced circulating corticosterone concentrations in both photoperiod treatments. Taken together, these results suggest that social environment may have important modulatory effects on photoperiod-regulated immune responses in male white-footed mice.