Incubation of homograft sensitized lymphoid cells with ribonuclease inhibits their ability to produce the “transfer reaction” on intradermal injection into the antigen donor. Ribonuclease in the concentration used does not kill the cells as determined by trypan blue staining and culture in Millipore filler chambers. The data indicate that intact protein synthesis of transferred immunologically active cells is essential for the expression of the “transfer reaction.