Most adenocarcinomas express altered MUC1 as a tumour-associated antigen. Due to suboptimal glycosylation in tumour-associated MUC1, the apomucin core is exposed, revealing new epitopes for antibody-directed immunotherapy. The human PH1 Fab binds specifically to this MUC1 apomucin. We describe the engineering and functional characterization of bi- and trivalent recombinant antibody derivatives from the PH1 Fab. Bi- and tribodies were made using the disulfide-stabilized Fab fragment as a heterodimerization scaffold with PH1 single-chain variable fragments fused to either one or both Fab-chain C-termini. Immunoassays revealed 27- and 165-fold improved dissociation constants (K(D) = 30 and 5 nM) of the PH1 bi- and tribodies compared with the parental Fab (K(D) = 820 nM). Unexpectedly, major differences were seen in the ability of the antibody constructs to bind shed and tumour cell-tethered MUC1. While the tribody did not discriminate between both MUC1 forms, the bibody demonstrated preferential interaction with membrane-bound MUC1 compared with shed MUC1. This preferential recognition of membrane-bound MUC1, along with the high serum stability of the bibody, its intermediate size and efficient internalization by MUC1(+) cells, makes the human PH1-derived bibody a valuable candidate as a cancer-targeting therapeutic.