Since the first appearance of this Manual in 1915, there have occurred some developments of fundamental importance in our conceptions of microbiological technic and laboratory procedure, as a consequence of which a revision of the text has become almost obligatory. Nearly all of our associates in the laboratory were connected with some branch of the Medical Corps during the War, and in their various capacities gained new viewpoints of certain phases of microbiological laboratory operations. Constant use of the Manual in our own laboratory and suggestions from a great number of sources, kindly furnished by teachers and even laboratory workers in various parts of the country, have induced us to attempt a revision with numerous corrections and many additions. The use of Fuller's Scale has been abandoned; the use of the autoclav rather than flowing steam in sterilization is recommended; the more recent views regarding reaction of media involving the determination of pH have been given consideration; the sections dealing with soil and water and sewage bacteriology have been almost completely rewritten; important alterations and additions have been made to the section on pathogenic bacteriology; vital additions have been made to the Appendix. It has been thought advisable to include in the pages preceding the first exercise a list of apparatus required by each student, also an outline or schedule for the use of the Manual in the beginning classes. Extensive changes in references have been made. New literature and new editions of texts demand that the instructor keep in touch himself and put his students in touch with the changes.