Our aim was to identify new genes involved in the cellular aspects of defense mechanism of Drosophila, as well as in melanotic tumor formation processes that are linked to blood cell disregulation. We have screened 1341 enhancer detector fly lines for expression of the lacZ reporter gene in larval hemocytes at the end of the third instar. We have selected 21 lines in which we observed a reproducible lacZ expression in blood cells. These lines were classified according to the subsets of hemocytes in which lacZ was expressed, and we identified five lines that can be used as lamellocyte markers. Three lines were selected for further analysis. The first exhibited strong lacZ expression in all lamellocytes. The second expressed lacZ in plasmatocytes and lamellocytes, and exhibited a melanotic tumor phenotype in larvae homozygous for the insertion. A third line showed a striking insertion-linked phenotype of melanized lymph glands (the hematopoietic organ), which resulted in the total absence of circulating hemocytes in the mutant larvae. We anticipate that this mutation, which we named domino, will prove a useful tool in the analysis of the role of hemocytes during the various aspects of immune response and melanotic tumor formation.