Abstract: Bifunctional chemicals useful for linking aldehyde containing moieties to moieties containing functionalities which are or can be reactive with sulfhydryl are claimed and have the formula:L--S--(CH.sub.2).sub.n1 --CONH--SPACER--NH--XwhereinL is a leaving group selected from --H or --S--Ar, wherein Ar represents optionally substituted phenyl or pyridyl;n.sub.1 =2-4;X is selected from: ##STR1## wherein Y is alkylene or oxaalkylene and Z is alkylene or a polypeptide residue briding the N-terminal amino group and C-terminal carboxy group thereof;the SPACER is oxaalkylene or oxaalkylene substituted with hydroxyl and specifically includes residues having formulas selected from--(CH.sub.2).sub.n2 --O--(CH.sub.2).sub.n2 --O--(CH.sub.2).sub.n2 --a)wherein each n2 is independently 2-4 and ##STR2## wherein n3 is 2-6. Also claimed are hydrazides, semicarbazides, and thiosemicarbazides of .omega.-mercaptocarboxylic acids which lack SPACER diamine.