When Toxoplasma tachyzoites were inoculated into normal kidney cell monolayers, they multiplied in the cells and further causing cell rupture. However, when Toxoplasma immune lymphokines (LKs) was added to normal kidney cells, the intracellular multiplication of the tachyzoites was inhibited remarkably; particularly, in the case in which the tachyzoites were exposed to immune fresh serum prior to inoculation into the cells. Toxoplasma growth inhibitory factor (Toxo-GIF) in LKs, with a m.w. of 30,000 to 40,000 was found to be contained in LKs-II or MIF-I fraction after Sephadex G-100 gel filtration. LKs collected from Balb/c mice inhibited tachyzoite multiplication in ICR-JCL normal mice kidney cells, suggesting that LKs activity did not show mouse-strain specificity. In contrast, the addition of Toxoplasma immune serum to infected cells had no effect on the intracellular multiplication of Toxoplasma and also did not prevent cell-to-cell spread. However, when extracellular tachyzoites were exposed to Toxoplasma immune fresh or heat-inactivated immune serum, especially to the former, at 37 degrees C for 30 min, their active penetration into the kidney cells were extremely inhibited and their subsequent multiplication in the cells were almost totally inhibited within 24 h as compared with those exposed to normal serum.