The present invention concerns compositions and methods of use of a humanized Class III anti-CEA antibody, comprising the heavy and light amino acid sequences SEQ ID NO:1 and SEQ ID NO:2. The antibody is effective to treat CEACAM5-expressing tumors, either alone or in combination with one or more therapeutic agents. Drug conjugated Class III anti-CEA antibodies, such as SN-38 or P2PDox immunoconjugates, are particularly efficacious. Surprisingly, the antibody-drug conjugates (ADCs) exhibit high anti-cancer efficacy, while exhibiting low levels of systemic toxicity that are readily treated with standard amelioration techniques. Antibodies and/or immunoconjugates comprising the amino acid sequences SEQ ID NO:1 and SEQ ID NO:2 are surprisingly efficacious for therapy of solid tumors, even when the tumor has proven resistant to standard anti-cancer therapies.