Compounds of the formula ##STR1## in which R.sup.1 denotes hydrogen or a saturated or singly or multiply unsaturated alkyl radical having up to 50 carbon atoms,X represents --CH.sub.2 --, --O-- or --NH--,R.sup.2 denotes hydrogen or a saturated or singly or multiply unsaturated alkyl radical having up to 50 carbon atoms,R.sup.3, R.sup.4 and R.sup.5, independently of one another, denotes hydrogen or acyl--CO--R.sup.6, R.sup.6 being an alkyl radical having up to 10 carbon atoms,R.sup.7 denotes hydrogen, C.sub.1 -C.sub.7 -alkyl-, hydroxymethyl, 1-hydroxyethyl, mercaptomethyl, 2-(methylthio)-ethyl, 3-aminopropyl, 3-ureidopropyl, 3-guanidyl-propyl, 4-aminobutyl, carboxymethyl, carbamylmethyl, 2-carboxyethyl, 2-carbamoylethyl, benzyl, 4-hydroxybenzyl, 3-indolylmethyl or 4-imidazolylmethyl, andR.sup.8 represents hydrogen or methyl, andR.sup.9 represents hydrogen, acetyl, benzoyl, trichloroacetyl, trifluoroacetyl, methoxycarbonyl, t-butyloxycarbonyl or benzyloxycarbonyl andR.sup.7 and R.sup.8 together can denote --CH.sub.2 --CH.sub.2 --CH.sub.2, and pharmaceutically acceptable salts thereof stimulate the immune system and can be used in conjuction with antibiotics and vaccines. New intermediates are also shown.