The capacity to induce complement-mediated cell lysis is greatly enhanced by truncating the hinge of IgG3 through exon deletions. This was shown by establishing five new cell lines which secreted chimeric IgG3 molecules with specificity for the hapten 4-hydroxy-3-nitrophenacetyl (NP) and having 47,45,32,15, and 0 amino acid hinge regions (the wild-type IgG3 has 62 amino acids in the hinge). Efficient complement activation and complement-mediated cell lysis did not depend on a long total hinge or on a long 'upper' hinge (the stretch from the beginning of the hinge to the first inter-heavy chain S-S bond). On the contrary, the mutant having a 15 amino acid hinge element was up to 10 times more efficient in complement lysis than the wild type. Thus the complement-activation potential appeared to be down-regulated in the wild type. On the other hand, the mutant lacking the hinge altogether did not activate complement or induce complement-mediated cytolysis. These findings have to be taken into account when antibodies are designed for human therapy.