Human lymphotoxin was genetically conjugated to the constant region of a human gamma 1 immunoglobulin gene at the end of either the second (CH2-LT) or third (CH3-LT) constant region domain. The altered heavy chain constant regions were combined in a plasmid vector together with the variable regions of a mouse anti-ganglioside GD2 antibody 14.18 and the human kappa constant region. The resulting immunoconjugate constructs were expressed in transfected hybridoma cells and tested for both their antibody and lymphotoxin activities. The two constructs were assembled to varying degrees depending on whether the third heavy chain constant region was present. Both forms retained their ability to bind antigen and mediate ADCC but only CH3-LT was able to mediate the lysis of melanoma target cells in the presence of human complement. Lymphotoxin activity, as defined in a cytolytic assay with mouse fibroblasts, was found to increase significantly as a function of heavy chain assembly and to be equivalent to unconjugated lymphotoxin. Neither of the constructs were cytotoxic for antigen-bearing melanoma cells that are normally resistant to lymphotoxin and tumor necrosis factor alpha. Such immunoconjugates may prove useful in targeting cytokines to the site of antigen-bearing cells in vivo. In this case, as a means of eliciting an inflammatory response at the site of a solid tumor.