NEW MATERIAL:A compound expressed by formula I (m represents a positive integer).

EXAMPLE: 2-Acetoxy-3-(7-carboxyheptyloxy) propyl 2-trimethylammonioethyl phosphate.

USE: An antigen useful for preparation of an antibody specific to a platelet activating factor (PAF), which antibody is useful for an assay of PAF.

PREPARATION: A compound expressed by formula II (x represents hydroxyl- protecting group; Ph represents phenyl) is reacted with a compound expressed by the formula Z-(CH2)m Y (Z represents Br, I, etc.; Y represents cyano, 2- oxazolinyl, etc.), followed by deprotection of the product and esterification of group Y thereof. Then the produced compound expressed by formula III (R represents 1W4C alkyl) is reacted with a compound expressed by formula IV, and the product is reacted successively with water and triethylamine. The obtained compound expressed by formula V (Me represents methyl) is then successively subjected to hydrolysis, hydrogenolysis and acetylation.