Daunorubicin substituted at N-3' with a benzyloxycarbonyl group (self-immolative spacer) linked to an alpha-D-galactosyl residue such as 7a and 7b have been prepared as prodrugs. Conversion of 7a and 7b to daunorubicin will be mediated by an immunoconjugate consisting of an alpha-D-galactosidase enzyme covalently attached to a tumor specific monoclonal antibody.