According to the strain used, spleen cells from mice injected two weeks before with 3 mg BCG, when they are engaged in a graft-versus-host reaction show a decrease in reactivity (C57Bl/6or AKR), an increase (DBA/2, BALB/C or SJL/J) or no modification (CBA, C3H). The in vitro responsiveness to PHA and conA of the treated spleen cells was lower than controls for all the strains studied. However, nylon column purified T cells from BCG treated mice showed a dissociated response, being less stimulated than normal cells by PHA and more so by ConA.