We identified conserved molecules (enzymes, peptides, cytokines) that might play a role in invertebrate innate immunity. We found these molecules by immunoserological and immunohistochemical methods in association with coelomocytes, leukocytes located in the coelomic cavity of the earthworm Eisenia foetida. We detected the enzyme Cu-Zn-superoxide-dismutase (SOD), cytokines (tumor necrosis factor-alpha, TNFalpha; transforming growth factor-alpha, TGFalpha; and alpha peptide hormone, thyreotrope stimulating hormone, TSH) in earthworm coelomocytes with monoclonal antibodies developed originally against human and/or mouse antigens. Three coelomocyte subpopulations were identified according to their form, size and granularity by microscopic and flow cytometric analysis. These cell populations showed different reactivity with antibodies against mammalian cell surface (CD) markers and different intracellular antigens. Two coelomocyte types showed cell surface positivity with anti-Thy-1 (CD90), CD24 and TNF-alpha antibodies. Strong cytoplasmic reaction was shown with anti-TNF-alpha and anti-SOD mAbs and a weaker but unambiguous reaction with thyroid stimulating hormone (TSH) in two cell populations. The third population was negative for all of the monoclonal antibodies. Our flow cytometric results were confirmed by confocal microscopy both on the cell surfaces and intracellularly.