Background: Lack of Gal expression on pig cells is associated with a reduced primate humoral immune response as well as a reduction in cytokine production by human cells in vitro. We investigated whether lack of Gal expression is associated with reduced human T-cell response in vitro. Methods: Peripheral blood mononuclear cells (PBMCs) were obtained from healthy humans and naÃ®ve baboons. Human CD4+ and CD8+ T cells were isolated. Porcine aortic endothelial cells (pAECs) were isolated from wild-type (WT) and α1,3-galactosyltransferase gene-knockout (GTKO) pigs. WT pAECs were treated with α-galactosidase, reducing Gal expression. Swine leukocyte antigen (SLA) class I and II expression on pAECs was measured, as was T-cell proliferation and cytokine production in response to pAECs. Results: Reduced Gal expression on WT pAECs after α-galactosidase treatment was associated with reduced human PBMC proliferation (P < 0.005). SLA class I and II expression on WT and GTKO pAECs was comparable. Human CD4+ and CD8+ T-cell proliferation was less against GTKO pAECs before (P < 0.001) and after (P < 0.01 and P < 0.05, respectively) activation. Human and baboon PBMC proliferation was less against GTKO pAECs before (P < 0.05) and after (P < 0.01 and P < 0.05, respectively) activation. Human PBMCs produced a comparable cytokine/chemokine response to WT and GTKO pAECs. However, there was less production of IFN-γ/TNF-α by CD4+ and IFN-γ/granzyme B/IP-10 by CD8+ T cells in response to GTKO pAECs. Conclusions: The absence of Gal on pig cells is associated with reduced human T-cell proliferation (and possibly selected cytokine production). Adaptive primate T-cell responses are likely to be reduced in GTKO xenograft recipients. © 2012 John Wiley & Sons A/S.