The in vitro and in vivo cultivation of horseshoe crab amebocytes is studied. Partial bleeding promotes the proliferation of horseshoe crab amebocytes, and could be a useful method for amebocyte production in vivo. The optimal conditions of in vitro cultivation of amebocytes were incubation at 15 degrees C and pH delta 6.8-7.2. The best medium was the hemolymph of the horseshoe crab. Measuring DNA synthesis of amebocytes by 3H-thymidine uptake showed that no proliferation of amebocytes occurs during in vitro cultivation. Because some protozoa were found in the hemolymph, primaquine, quinine, pyrimethamine and metronidazole were used in the culture medium. Primaquine and metronidazole have a protozoacidal effect on the protozoa parasitising in the horseshoe crab.