An erbB2 antibody is provided that binds preferentially to disease cells having an erbB2 density greater than a normal erbB2 density. The erbB2 antibody comprises a heavy chain and a light chain. Each chain has a constant region and a variable region. Each variable region comprises framework regions and complementarity determining regions (CDRs), wherein the CDRs have an amino acid sequence set forth below: For the heavy chain: CDR1 GFNIKDTYIH (SEQ ID No. 1) CDR2 RIYPTNGY57TR59 YADSVKG (SEQ ID No. 2) CDR3 WGGDGFYAMDY (SEQ ID No. 3) For the light chain: CDR1 RASQDVN30TAVA (SEQ ID No. 4) CDR2 SASF53LYS (SEQ ID No. 5) CDR3 QQHY92TTPPT (SEQ ID No. 6). At least one of Y57, R59, N30, F53, and Y92 is substituted by an amino acid that confers on said antibody a reduced erbB2 binding affinity (Kd) that is in the range from 0.1 nM to 100 nM. The substitution is other than N30A, F53N, Y92A and Y92F when there is a single substitution in the antibody light chain.