In Drosophila, defense against foreign pathogens is mediated by an effective innate immune system, the cellular arm of which is composed of circulating hemocytes that engulf bacteria and encapsulate larger foreign particles. Three hemocyte types occur: plasmatocytes, crystal cells, and lamellocytes. The most abundant larval hemocyte type is the plasmatocyte, which is responsible for phagocytosis and is present either in circulation or in adherent sessile domains under the larval cuticle. The mechanisms controlling differentiation of plasmatocytes and their migration toward these sessile compartments are unclear. To address these questions we have conducted a misexpression screen using the plasmatocyte-expressed GAL4 driver Peroxidasin-GAL4 (Pxn-GAL4) and existing enhancer-promoter (EP) and EP yellow (EY) transposon libraries to systematically misexpress approximately 20% of Drosophila genes in larval hemocytes. The Pxn-GAL4 strain also contains a UAS-GFP reporter enabling hemocyte phenotypes to be visualized in the semitransparent larvae. Among 3412 insertions screened we uncovered 101 candidate hemocyte regulators. Some of these are known to control hemocyte development, but the majority either have no characterized function or are proteins of known function not previously implicated in hemocyte development. We have further analyzed three candidate genes for changes in hemocyte morphology, cell-cell adhesion properties, phagocytosis activity, and melanotic tumor formation.