Recent evidence has shown that under in-vitro conditions human seminal plasma can interfere directly or indirectly with the function of cells of the immune system. It is however, questionable whether the results generated in vitro can be related directly to in-vivo activity. We have therefore standardized an in-vivo immunobioassay to detect the immunosuppressive property of human seminal plasma using adoptive transfer of contact sensitivity to a specific antigen such as dinitrofluorobenzene. Our results indicate that when sensitized lymphoid cells were incubated in vitro with human seminal plasma, their ability to transfer the delayed hypersensitivity in non-sensitized mice was suppressed or inhibited in comparison with the controls. The percentage suppression varied with different samples but the results indicate clearly that the immunosuppressive properties of human seminal plasma can be demonstrated using an in-vivo immunoassay.