Thirteen laboratories engaged in cattle blood typing have participated in tests to compare their blood-typing reagents. Over 75,000 individual tests of cattle erythrocytes were made. When the laboratories tested duplicate samples without knowing they were duplicates, the results were in agreement 99 percent of the time. Tests on samples from the same 199 animals with 56 different reagents from two or more of the laboratories showed agreement 89 percent of the time. The agreement was unexpectedly good, since the primary purpose of the tests was to allow comparison of independently developed reagents and since the sources of error were numerous.