For the first time, the fine structure of the hemocytes from the horseshoe crab Tachypleus tridentatus is investigated by transmission electron microscopy and light microscopy serial sectioning. Two morphologically distinct, ellipsoidal, and mononucleate hemocytes--granulocytes (amebocytes) and plasmatocytes--are revealed. Granulocytes constitute about 97% of the hemocytes. They have a marginal band of microtubules, a heterochromatic nucleus, distended but poorly developed RER, few free ribosomes, few mitochondria, and many large secretory granules. The majority of these granules have a uniform content and are mature. Structured granules located in the proximity of Golgi complexes may be immature transitional stages leading to the mature uniform granules. Upon stimulation with endotoxin from gram negative bacteria, the mature granules become transitory structured before exocytosis. In contrast, the immature granules are not exocytosed. Plasmatocytes constitute about 3% of the hemocytes. They differ from granulocytes by having an euchromatic nucleus, a well-developed RER of flattened or tubular cisternae, many free ribosomes, many mitochondria, but only few, if any, large secretory granules. Apparently, plasmatocytes are not affected by endotoxin. The relationship and possible functions of granulocytes and plasmatocytes are discussed and compared with those of the horseshoe crab, Limulus polyphemus.