A novel amino acid derivative represented by the general formula (I) shown below or a salt thereof have now been provided by chemical synthesis, and it has been found that these new compounds stimulate proliferation and physiological activities of the T cells of mammals, have an immunomodulating activity and also have an antitumor activity or a carcinostatic activity. It has also been found that these new compounds have an effect of stimulating the proliferation of hematopoietic stem cells. ##STR1## wherein y is 0 or 1; R.sup.1 represents an unsubstituted C.sub.1 -C.sub.8 alkyl group or others; R.sup.2 represents a hydrogen atom, a C.sub.1 -C.sub.5 alkyl group or others; R.sup.3 represents a hydrogen atom, a C.sub.1 -C.sub.8 alkyl group or others; R.sup.4 represents a hydrogen atom, a C.sub.1 -C.sub.3 alkyl group or others; R.sup.5 represents a group --OR.sup.42, a group ##STR2## or others.