Rabbit lymphocytes, settled up in microcappillary tubes migrate into culture medium. In the previous communication we have shown that migration of mixture of lymphocytes derived from two blood donors was significantly inferior in 73% of cultures as compared to the corresponding monocultures. 4% of mixed cultures exhibited significant enhancement of migration and in 23% no effects were seen. Use of lymphocytes from rabbits 4-6 days after kidney allotransplantation and their corresponding donors resulted in conversion of this ratio. Percentage of cultures exhibiting enhancement of migration (production LyMSF) rose to 39, 46% of cultures were inhibited (produced LyMIF) and 15% did not show any significant effect. Treatment of kidney recipients with ATG exerted strong inhibitory effect on the appearance of LyMSF and on the appearance of LyMIF in mixed cultures. These results suggested that LyMIF and LyMSF were produced by separate cell populations.