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A variety of known or suspected inducers of crisis form parasites in cultivated Plasmodium falciparum were examined. Sera from Sudanese residents of malaria-endemic areas, sera from American tuberculosis patients, and rabbit sera containing tumor necrosis factor were assayed in vitro for cytotoxic activities against P. falciparum and mouse L-M cell cultures. Inhibition was determined by measurement of incorporation of radiolabeled nucleic acid precursors. When compared to normal serum, parasites grown in the presence of a 1:4 dilution of rabbit sera containing tumor necrosis factor, TB patient sera, or Sudanese sera were metabolically inhibited 73%, 75%, and 95%, respectively. However, only the rabbit sera containing tumor necrosis factor were cytotoxic to L-M cells, inhibiting radiolabel incorporation by 80% at a 1:1,000 serum dilution. These findings suggest that tumor necrosis factor is apparently not responsible for the induction of parasite crisis forms by the inhibitory human sera tested. In addition, human gamma-interferon had no effect on parasite growth.