There is no doubt that a famlly of cyclophillns (CyPs), which exist abundantly among prokaryotics and eukaryotics, do have an unique and essential role to play for cell cycle. Eventhough several researchers have proposed that CyPs were to be chaperonial enzymes in view of their apparent PPlase-activity, what CyPs actually do yet remain a mystery. In connection to this, one will concentrate on very recently obtained structural information on the complex of E.coli cytoplasmic CyP and tripeptide substrate-mimicry, togather with yeast cytoplasmic CyP. The implications of this sturucture in the plausible mechanism for peptidal substrate binding on CyP is also discussed.