Grafting of donor mammalian testis tissue into recipient mice allows completion of spermatogenesis in the grafted tissue and therefore can serve as a new option for preservation of male germ line. For testis tissue xenografting, castrated male nude mice typically serve as recipients, each receiving 8 testis tissue fragments; however, no study has comprehensively investigated donor and recipient factors. The objective of this study was to determine the effects of strain of immunodeficient recipient mouse (nude v. SCID), gonadal status (intact v. gonadectomized), and gender (male v. female) on the outcome of testis tissue xenografting. A secondary objective was to determine the optimal number of testis tissue fragments per mouse most suitable for xenografting. Testis parenchyma from newborn piglets were cut into small fragments (5 mg each) and grafted under the back skin of different groups of immunodeficient mice. In Experiment 1, 8 groups of mice (n = 7/group) served as recipients: castrated male nude, intact male nude, ovariectomized female nude, intact female nude, castrated male SCID, intact male SCID, ovariectomized female SCID, and intact female SCID. In Experiment 2, 4 groups of mice (n = 10/group) served as recipients of 2, 4, 8, or 16 testis tissue fragments per mouse. Recipient mice were sacrificed 8 months after grafting and the weight of the grafts and vesicular glands (male mice) were compared among groups by analysis of variance. In Experiment 1, mouse gonadal status (intact v. gonadectomized) did not affect the total graft weight (P > 0.05), but both the recipient mouse strain (nude v. SCID) and gender (male v. female) affected the total graft weight (2460 ± 320.9, 1420 ± 290.0, 758 ± 156.7, and 2780 ± 297.4, mean ± SEM, P < 0.0001 for SCID, nude, female, and male mice, respectively). In Experiment 2, the total graft weight was highest in the group of mice receiving 8 testis tissue fragments (192 ± 76.1, 695 ± 96.5, 2443 ± 338.8, and 1458 ± 305.4, mean ± SEM, P < 0.0001 for 2, 4, 8, or 16 fragment groups, respectively). These results collectively indicate that male SCID mice receiving 8 testis tissue fragments provide optimized conditions for the recovery of largest grafts.