Two peptide forms of the antitumor transition state analogue N-phosphonoacetyl-L-aspartic acid (N2-phosphonoacetyl-N4-glycylglycinamidoethyl-L-asparagine and N1-glycylglycinamidoethyl-N2-phosphonoacetyl-L-isoasparagine ) have been synthesized to obtain potential medicinal agents useful as prodrugs or in a lysosomotropic carrier approach. The bridging unit, ethylenediamine, used for synthetic purposes might be of general interest.