A novel thiazolidine derivative having the following general formula ##STR1## wherein R.sup.1 and R.sup.2 are each lower alkyl having 1 to 3 carbon atoms, or R.sup.1 and R.sup.2 together with the nitrogen atom to which they are attached form a heterocyclic ring selected from the group consisting of morpholino, piperazino, piperidino and pyrrolidino, or said heterocyclic ring substituted with 1 or 2 methyl groups, and the pharmaceutically acceptable salts thereof are useful immunomodulators.