The present invention relates to targetable constructs which may be bound by a bi-specific antibody or antibody fragment having at least one arm that specifically binds construct. The targetable construct comprises a carrier portion which comprises or bears at least one epitope recognizable by at least one arm of said bi-specific antibody or antibody fragment. The targetable construct further comprises one or more therapeutic or diagnostic agents or enzymes. The invention provides constructs and methods for producing the targetable constructs and bi-specific antibodies or antibody fragments, as well as methods for using them.