We demonstrated, with the aid of PRP, that precursors of helper T cells in the humoral immune response to SRBC belong to a minor thymocyte subset bearing the: Thy-1 +/-, H-2+, L3T4-, lyt 2-, CD3- phenotype. We showed, in addition, that PRP induced among the double negative (L3T4-, lyt 2-) thymocytes an appearance of differentiation antigens such as: L3T4, lyt 2, CD3 and T-cell receptor (alpha/beta). The data presented in this paper are in accordance with the present view on the process of T-cell maturation.