Peptide constructs were engineered by colinear synthesis of two short synthetic peptide determinants: a determinant recognized by T helper cells (TD,) and a determinant recognized by T cytotoxic cells (TD,). Three types of constructs were synthesized: TD,-TDr, , TD,-TD, , and TD,,-KK-TD,, where KK are two lysine residues. In vivo immunization with free construct induced cytolytic lymphocytes (CTL) only in the case of TD,-TD,,. However, immunization with spleen cells to which these constructs had been internalized by hypertonic shock, induced CTL activity in all three cases. No CTL could be induced after immunization with free TD, in either protocol. These results indicate that cell internalization of the construct might be essential for CTL induction, and also, that "help" from the TDr, seems to be required. o 1992 Academic PKSS, IX.