Anti-Lewis Y antibodies are described. Methods for preparing monomeric cytotoxic drug/carrier conjugates with a drug loading significantly higher than in previously reported procedures and with decreased aggregation and low conjugate fraction (LCF) are described. Cytotoxic drug derivative/antibody conjugates, compositions comprising the conjugates and uses of the conjugates are also described. Specifically, monomeric calicheamicin derivative/anti-Lewis Y antibody conjugates, compositions comprising the conjugates and uses of the conjugates are also described.