Abstract: A novel substance, that is a cyclic analogue of a naturally-occurring phagocytosis-stimulant peptide - threonyl-cyclo-[-N.sup..epsilon. -lysyl-prolyl-arginyl-] of the formula: ##STR1## A method for preparing threonyl-cyclo-[-N.sup..epsilon. -lysyl-prolyl-arginyl-] by way of a step-wise building-up of the peptide chain from the C-terminal by means of activated ethers of benzyloxycarbonyl-proline, tert.butyloxycarbonyl- -N.sup..epsilon. -benzyl-oxycarbonyl-lysine and tert.butyloxycarbonyl-threonine, followed by cyclization of the resulting partly blocked tetrapeptide using Woodward reagent in an excess of dimethylformamide and isolation of the desired product.

Abstract: A synthetic analog of tuftisin, namely: glycyl-lysyl-prolyl-arginine of the formula: ##STR1## The synthetic analog of tuftisin according to the present invention has a high phagocytosis-stimulating activity when used in small doses.

Abstract: Novel cyclic analogues of callidine have the following general formula: ##STR1## wherein Pro is moiety of L-proline, Gly--that of glycine, Phe--phenylalanine, Arg--L-argenine; R is a residue of .alpha.- or .omega.-aminoacid or peptide; R.sub.1 is a residue of .alpha.- or .omega.- amino-acid; R.sub.2 is L-serine or glycine, the carbonyl group of arginine being connected either with R or with R.sub.1 by means of a covalent peptide bond using the .omega.-amino group of the aminoacid located in the 1 or 2 position of the calladine sequence. The novel compounds according to the present invention possess a prolonged hypotensive effect of selective character.

Abstract: Cyclic analog of bradykinin having the formula ##STR1## where Lys-L-lysine residue, Pro-L-proline residue, Gly-glycine residue, Phe-L-phenylalanine residue, Arg-L-arginine residue. The present compound can create prolonged depressor effect in experiments in vivo, as well as increase vascular permeability in experiment in situ less than bradykinin. In experiments in vitro this compound displays no myotropic effect.