linking agent which is a compound having at least two functional groups one of which bonds to a functional group of the biologically active protein and the other of which bonds to the functional group of the first chemical linking agent the first chemical linking agent preferably has a carboxyl group cooh or an amino group nh 2 as a functional group the lecithin derivative having such a first chemical linking agent bonded thereto is preferably the one represented by the following formula iii str2 in the above formula iii r 1 is a linear or branched alkylene group having from 1 to 24 carbon atoms preferably a linear alkylene group having from 2 to 10 carbon atoms z 39 is a reactive functional group preferably a carboxyl group or an amino group z 39 may be a functional group capable of being converted to a carboxyl group such as a temporarily protected carboxyl group an amino group a haloformyl group or an alkoxycarbonyl group a compound of the formula iii wherein z 39 is a carboxyl group or a group capable of being converted to a carboxyl group may be prepared by e g a method wherein an acid anhydride of the formula str3 is reacted to lysolecithin of.