The mixed lymphocyte culture (MLC) test has been regarded as an in vitro model of the recognition or sensitization phase of the homograft or graft-versus-host reaction. It has been suggested that the graft-versus-host response in vivo is less in xenogeneic combinations than in allogeneic ones and that there is a similar quantitative relationship in MLC responses. Given the above interpretation of the MLC test, this could suggest that the lesser reactivity in xenogeneic combinations may be due to a lesser recognition of the stimulus. We have done nine experiments testing allogeneic and xenogeneic combinations in MLC, largely in combinatorial fashion. The results indicate that the response in xenogeneic MLC may be as great as that in allogeneic MLC and that, as in different allogeneic mixtures, there is great variation in the extent to which xenogeneic mixtures may respond.

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