BMPH, EMCH and KMUH are heterobifunctional crosslinkers containing sulfhydryl-reactive maleimide and carbonyl-reactive hydrazide moieties. Maleimides react with free sulfhydryls (-SH) to form stable thioether bonds. Hydrazide groups react with carbonyls (aldehydes and ketones) to form stable hydrazone bonds. Aldehyde groups can be created by periodate-oxidation of sialic acid and other sugar components of glycoprotein polysaccharides. Thus, these crosslinkers are useful for conjugating glycoproteins and sulfhydryl-containing peptides or proteins. Alternatively, the amine of the hydrazide moiety can be reacted to carboxyl groups using the crosslinker EDC.