NEW MATERIAL:The compound of formula I [R1 is 5-6-membered heterocyclic group which may be condensed with benzene ring; R2 is H, alkyl or R1; X1 is O, S or CH2; Y1 is O, S or NR4 (R4 is H, alkyl or carboxy); A1 is methylene or ethylene; R3 is NR5R6, group of formula II, formula III (one of R5 and R6 is H or hydrocarbon group and the other is hydrocarbon group or R1; A2 and A3 are alkylene; Z is CH or N; R7 is H, hydrocarbon group, carboxy, acyl, carbamoyl, etc.), etc.] or its salt.

EXAMPLE: [N-tert-butoxycarbonyl-2-(3-pyridyl) thiazolidine-4-carbo-nyl]-L-methio- nine methyl ester.

USE: A drug composition having antagonistic action against platelet activating factor.

PREPARATION: The compound of formula I can be produced by reacting a compound of formula IV [R12, R13 and Y2 are (protected) R1, R2 and Y1) with a compound of formula H-R14 [R14 is (protected) R3] and, as necessary, removing the protecting group.