METHODS Strains of Staphylococci A number of strains of staphylococci, both coagulase-positive and negative, were classified into 5 groups, as follows:(1) 60 coagulase-positive strains from human pyogenic infections,(2) 10 coagulase-positive strains from nasal carriers,(3) 5 coagulase-positive strains from cases of food poisoning,(4) 12 coagulase-positive strains from animal sources, and (5) 23 coagulase-negative saprophytic strains. Most of the work was done with strain V8 from group 1, received from Dr. FC 0. Valentine of the London Hospital. It was isolated from a case of chronic furunculosis of 5 years' standing, was deficient in pigment, and produced high titres of PV leucocidin.

Production of Leucocidin For production of culture filtrate on the scale of litres the aerated column device of van Heyningen and Gladstone (1953a) was used. This supplied 4 1. of culture every 24 hr. For small-scale experiments (10 ml.) a modification of Monod's rocking T-culture tube was used (van Heyningen and Gladstone, 1953b).