+ More than half a century ago, Kraus and Ludwig 1 made the interesting observation that certain bacteria, namely, staphylococci and vibrios agglutinate red blood cells. These studies on bacterial hemagglutination were extended during the following decades. However, it was not until after 1941, when the classical investigations on virus hemagglutination were published by Hirst 2 and by McClelland and Hare,3 that renewed interest in bacterial hemagglutination developed. As a result of these studies, it is now possible to recognize two basically different types of bacterial hemagglutination. In the first type, the bacteria bring forth agglutination of suitable red blood cells; this type may be referred to as direct bacterial hemagglutination. In the second type, the bacteria cause a change of the red blood cells, rendering them susceptible