The antiproliferative activity of spirocyclic cyclodipeptides containing 1-aminocyclopentanecarboxylic acid (Acp) was investigated and compared with Pro-Leu-Gly-NH2 (MIF), cyclo(Gly-Leu) and cyclo (Sar-Sar), a compound with denaturing activity, employing the chick embryonic caudal morphogenetic system. Out of the compounds tested, MIF and cyclo(Acp-Ala) appear to exhibit the least inhibitory activity on growth.