exhibited antitumoractivity against L1210 leukemia and B-16 melanoma in mice at doses which were not toxic to the host. Structural analogues of 5-S-cysteinyl-3,4-dihydroxyphenylalanine including several new compounds, were synthesized and tested for growth inhibition of cultured cells of human neuroblastoma YT-nu and Chinese hamster fibroblasts Don-6. Some were also examined for antitumoractivity against