NEW MATERIAL:A compound expressed by formula I (R is lower alkylene; R' is H or carboxyl-protecting group).

EXAMPLE: Carboxymethyl(trans 1-propenyl)disulfide.

USE: An antitumor agent.

PREPARATION: A carboxylic acid thiol derivative expressed by formula II (A is thiol-protecting group eliminative with a halogenating agent) is reacted with a trans 1-propenethiol derivative expressed by formula III in the presence of a halogenating agent to afford the compound expressed by formula I. The reaction is carried out by using the compound expressed by formula II in an amount of normally 0.5-1.5 equiv. based on the compound expressed by formula III in the presence of an inert solvent (e.g., THF or dioxane).