A METHOD OF PREPARING AN IMMUNOCONJUGATE. A method is disclosed for preparing an immunoconjugate of a member of the family of calicheamicins with a monoclonal antibody as carrier, which comprises preparing a monoacylated hydrazide where the acyl group contains an S-protected mercapto function, removing the protecting group of the S-protected mercapto function, and using the resultant hydrazide for preparing said immunoconjugate