Abstract
A low level of T-lymphocyte-mediated cytotoxic activity was demonstrated after primary in vivo immunization in an H-2-compatible strain combination (B10.D2-DBA/2 [mice]). This phenomenon was demonstrated in a 2nd responder strain (BALB/c [mice]) and was not restricted to the B10.D2 genetic background. Results on F1 hybrid mice indicate that the M-locus determinant might be involved in this reaction. It is possible that it is involved in the afferent, efferent or both limbs of the reaction.