Rabbits were immunized both with lysozyme and with dinitrophenylated bovine serum albumin. No antibodies against the dinitrophenyl hapten were produced when cell suspensions of their spleens were exposed in vitro to dinitro-phenyl-ovalbumin, whereas a positive response was obtained with dinitrophenyl-lysozyme. Moreover, when spleen cells from rabbits previously immunized with dinitrophenyl-bovine serum albumin were exposed in vitro to bovine serum albumin alone, they produced antibodies against the dinitrophenyl hapten.