Hemocyte types involved in encapsulation were characterized using monoclonal antibodies (mAbs). This approach revealed that four hemocyte types in Pseudoplusia includens could be classified into two antigenically distinct cell lines. The first line comprised granulocytes (GR) and spherulocytes (SP) and the second line comprised plasmatocytes (PL) and oenocytoids (OE). One of the mAbs labeled a subpopulation of plasmatocytes that spread on culture plate surfaces. This subclass represented approximately 70% of all plasmatocytes. The cytoplasmic punctate staining of granulocytes clearly decreased upon short term culture, suggesting the associated antigens were released into the culture medium during cell spreading. A follow-up of protein secretion into culture medium by Western blotting confirmed this hypothesis for both granulocytes and plasmatocytes. In a few cases, discharged proteins exhibited a short half-life suggesting they may behave as regulatory molecules. Among them, plasmatocyte proteins of ± 25 kDa might be mobilized at an early stage of encapsulation. The same proteins appeared to accumulate at the periphery of the median plasmatocyte multilayer in late capsules. This location coincides with where an outer monolayer of granulocytes attaches and causes termination of capsule growth. These preliminary results raise the possibility that released proteins regulate hemocyte recruitment during encapsulation.