The clotting of the hemolymph of the cockroach,Leucophaea maderae (Blattaria) was studiedin vitro. Isolated hemolymph plasma and hemocytes were mixed under various conditions to analyze the effect of specific ions, inhibitors, and other factors on clotting. The extent of clotting was recorded by measuring the consumption of plasma coagulogen and the solubility of the resulting clot. 1. Clotting is strongly inhibited by EDTA; the inhibition can be reversed by Ca2+, Mn2+ or Zn2+, but not by other alkaline earth metal ions. 2. Clotting is also strongly inhibited by several SH-blockers (like iodoacetamide); no inhibition is observed with artificial or natural trypsin inhibitors, or with inhibitors of vertebrate fibrin crosslinking. The iodoacetamide-sensitive factor, presumably a SH-enzyme, is derived from the hemocytes; it might be identical with the hemocyte coagulogen. 3. Clotting inLeucophaea is a fast reaction: in less than 3 min the clot is completely insoluble. 4. The hemocyte coagulogen rapidly loses clotting activity after its release from the hemocytes. This ‘aging’ is correlated with, and possibly caused by the disappearance of free SH-groups. 5. The aging of the hemocyte coagulogen does not completely abolish its ability to interact with the plasma coagulogen. At low ionic strength an association of the two coagulogens can be observed which is reversed by higher ionic strength.