Abstract
Thirty-one Holstein cows were placed in pasture in Shimane Prefecture over a period from April to August, 1970. Each of them was inoculated with 1ml of defibrinated blood of indigenous cattle infected with piroplasrnas to a severity of -H-F, in April, 1970. As a result, piroplasmas were detected from all the cows by 45 days after inoculation. Finally, 13 cows (41.9%) were proved to have acquired immunity to the protozoa, without receiving any treatment during the proper period of grazing. In general, 70 days seemed to be insufficient and at least 4 months required for effective immunity to be established.
