Gangliosides are sialic acid-containing glycosphingolipids present in the plasma membrane of most mammalian cells. In humans, the expression of the N-glycolylated (Neu5Gc) variant of the sialic acid has been associated with malignant transformation, constituting therefore an attractive target for cancer immunotherapy. P3 monoclonal antibody (mAb) recognizes Neu5Gc-containing gangliosides, as well as sulfatides. Heavy chain CDR3 (H-CDR3) arginine residues have been shown to be crucial for ganglioside recognition, but less important for anti-idiotypic antibody binding. Here, we describe the effect on antibody reactivity of different mutations involving a single H-CDR3 acid residue. Substitution of glutamate 99 (Kabat numbering) by arginine, aspartate or serine residues resulted in no differences in anti-idiotype binding. However, the first mutation caused increased reactivity with the antigen, including a cytotoxic effect of the antibody on ganglioside-expressing cells previously unseen for the wild type antibody. Another antibody that recognizes N-glycolyl-GM3 ganglioside (GM3(Neu5Gc)), but not other glycolipids, named 14F7, exhibits also an arginine-enriched H-CDR3 and a complement-independent cell death activity. Unlike 14F7 mAb, the cytotoxicity of the P3 E(99)→R mutant antibody did not exclusively depend on ganglioside expression on tumor cells.