Cyclic dipeptides of the formula cyclo(-Acc-X-), where Acc = 1-aminocyclopropane-1-carboxylic acid, X = Gly, Ala, Val, Leu, Phe or Tyr, were synthesized. The prepared 2,5-piperazinediones showed no proliferative or antiproliferative activity on normal human embryo cells as well as on HL60 leukemia cells. Results of trace memory assays in mice were negative.