ABSTRACT: We investigated mouse testicular extract (TE) to clarify its biological functions in reproductive immunity. TE, at concentrations of 50–300 μg/ml, enhanced macrophage activities of spreading, glucose consumption, and cytostasis against a susceptible tumor cell line. On the other hand, TE inhibited concanavalin A (Con A)-induced T-cell blastogenesis in the dose range of 10–600 μg/ml. To elucidate the origin of TE, W/Wvmice, which genetically lack germ cells, were used. TE obtained from W/Wv mice enhanced the spreadability of macrophages and inhibited Con A-induced blastogenesis of T cells. The enhancement of macrophage spreading was only achieved by the interstitial fluid (IF), while the suppression of Con A-induced T-cell responses was detected in seminiferous tubule fluid (STF) as well as in IF. TE did not affect listerial antigen-specific responses of lymphocytes in vitro. These results suggest that TE has the capacity to regulate the biological responses associated with reproduction.