The streptococci offer an interesting and important field for study because among them are found harmless and even useful organisms as well as extremely virulent ones. Milk is perhaps the most important harborer of the harmless streptococci, but it may at times contain pathogenic forms. While milk cannot be considered a source of any micro-organism, it is an ideal food for most forms and may therefore contain many kinds of streptococci with which it has been contaminated.