When migration inhibitory factor (MIF) assay in vitro was conducted on the lymphokines (LKs), it was observed that the percentage of MIF activity was greatly increased from the 3rd to the 4th week postinfection. On the succeeding weeks there was a noticeable decrease in the MIF activity noted on the 8th week postinfection of Toxoplasma. MIF activity was examined at 1, 6, 12, 18, 24 and 48 hours in non-immune spleen cells as well as Toxoplasma immune spleen cells in the 2nd week after the final challenge inoculation. MIF activity in Toxoplasma immune spleen cells were 2, 21, 29, 54, 70 and 93 percentage, respectively. The MIF activity of hyperimmunized spleen cells produced an activity of approximately 50% at 18 hours as compared to the non-immune spleen cells. Characterization of the MIF was performed using Sephadex G-100 and DEAE Sephadex A 50. Two distinct peaks of MIF were identified and separated by Sephadex G-100 gel filtration, calculating molecular weights between 30,000 to 40,000 and 3,000 to 5,000, respectively. When the fast peak by Sephadex G-100 was eluted again in DEAE Sephadex A 50, the peak was separated into 4 units, all units showing MIF activity.