There are disclosed peptides having the following formula:A--X--Z--Y--Bwherein A is ##STR1## X is a suitable neutral, aliphatic or aromatic amino acid residue; e.g., one selected from the group consisting of ALA, 2-Me-ALA, GLY, LEU, ILE, LYS, THR, SER, PHE, MET, D-ALA, D-LEU, D-ILE, D-LYS, D-THR, allo-THR, D-SER, D-PHE, D-MET, and SAR; Z is ##STR2## Y is GLY, SER, THR, LEU, ILE, VAL, OR SAR; B IS TYR-R', D-TYR-R', decarboxy-TYR, or ##STR3## m is 3 or 4; n is 1, 2, or 3; R''' is hydrogen, C.sub.1 -C.sub.7 alkyl, C.sub.6 -C.sub.12 aryl, or C.sub.1 -C.sub.7 alkanoyl; and R and R' are substituents which do not substantially affect the biological activity of the peptides, provided that R-ARG-LYS-ASP-VAL-TYR-R' is excluded. These peptides have the capability of inducing the differentiation of T-lymphocytes but not of complement receptor (CR.sup.+) B-lymphocytes and thus are useful in a number of therapeutic areas. Also provided are derivatives of the peptides, therapeutic compositions, and methods for use of the compositions.