Using P element-mediated mutagenesis we have isolated 20 X-linked lethal mutations, representing at least 14 complementation groups, which exhibit melanotic tumor phenotypes. We present the systematic analysis of this interesting group of lethal mutations that were selected for their visible melanotic or immune response. The lethal and melanotic tumor phenotypes of each lethal(1) aberrant immune response (air) mutation are pleiotropic effects of single genetic lesions. Lethality occurs throughout the larval and early pupal periods of development and larval development is extended in some air mutants. The air mutant lethal syndromes include abnormalities associated with the brain, haematopoietic organs, gut, salivary glands, ring glands, and imaginal discs. Additional characterization of the melanotic tumor mutations Tuml and tu(1)Szts have indicated that the melanotic tumor phenotype is similar to that observed in the air mutants. These studies have led to the proposal that two distinct classes of melanotic tumor mutations exist. Class 1 includes mutants in which melanotic tumors result from "autoimmune responses" or the response of an apparently normal immune system to the presence of abnormal target tissues. The Class 2 mutants display obvious defects in the haematopoietic organs or haemocytes, manifested as overgrowth, and the resulting aberrant immune system behavior may contribute to melanotic tumor formation.