In the cockroach, Periplaneta americana (L.), hemolymph coagulation (a) is inhibited for as long as 30 minut es at 0° to 4°c, (b) is initiated at 5°c, (c) is permanently inhibited at 60°c, and (d) takes place in 6 distinct stages. Hemocyte agglutination and transformation is inhibited at 0° to 4°C, is permanently prevented at 55°c, and is independent of plasmal factors. Live plasmatocytes, granular hemocytes, and cystocytes are structurally identical, but differ functionally in their capacity to phagocytize and in their fragility. The cystocyte's primary function is the initiation of coagulation and/or precipitation of the plasma by ejecting cytoplasmic material, including mitochondria, into the surrounding plasma. Hemocyte-free plasma will not spontaneously precipitate, but requires either ionic calcium released from transforming hemocytes, and/or material from exploded cystocyte mitochondria . Substances inside mitochondria may well be t he source of a coagulation-inducing substance that initiates plasma precipitation and veil formation. Substances involved in P. americana coagulation are present in the plasma of 9 other species of cockroach which react to P. americana cystocytes. Substances in the plasma of Tenebrio molitor, Galleria mellonella, or Rhodnius prolixus do not precipitate in the presence of P. americana cystocytes. The amount and/or effectiveness of a coagulation-inducing substance released from cystocytes presumably determines the degree of plasma precipitation. Physiologically active substances contained in and/or released from the corpora allata and c. cardiaca, but lacking in the brain, may regulate the percentage of circulating cystocytes, thereby influencing the coagulability of the hemolymph.