Examined were 26 ill persons from two epidemic areas of trichinellosis, all of them showed positive leukergic reaction as early as in first examinations performed at 3 weeks of infection. Negative result was obtained in 29 another cases suspected but exhibiting no clinical symptoms. Among the latter group a single leukergic reaction was found sporadically in only 9 cases which may be attributed to asymptomatic course of infection or to other pathologic conditions. The greatest intensity of leukergy was seen at 3-6 weeks with subsequent decline and extinction at 13-26 weeks of infection. Similar, though less regular results were obtained in 30 rabbits, each infected with 1000-8000 larvae. In this case, a highly early onset of leukergic reaction could be seized at 6 and even 4 days of infection. No clearcut differences were, however, demonstrated in the intensity of the reaction depending on the size of the inoculum. Parallel examinations were concerned with white cell count, number of acidophilic leukocytes (in %) and complement fixation reaction. Leukocytosis, eosinophilia and positive serologic reaction were mostly found to follow leukergy. Leukergic reaction seems to be an additional, though unspecific, diagnostic sign of trichinellosis; in the present studies it appeared in 100% of cases soon after the moment of infection. It is possibly that leukergy has both practical and theoretical value in the field of trichinellosis.