2. The toxic agent is not inactivated by alcohol. 3. The toxic agent is not destroyed by drying fixed flies. 4. The toxic agent is not affected by glycerin, but is, if anything, more active in pastes made from the ground fly and glycerin, than in the ground flies as such. 5. The toxic agent is rendered inactive or destroyed by hydrochloric acid in a concentration of 0.25%. 6. The toxic agent is most abundant in the region of the anatomical structures connected with the biting and salivary apparatus (head and thorax). 7. The toxic agent is not affected by a 0.5% solution of sodium bicarbonate. 8. The toxic agent is not affected by exposure to dry heat at 100° C. for two hours. 9. The toxic agent is destroyed or rendered inactive in alkaline solution by a typical hydrolytic ferment, pancreatin. 10. Incomplete experimental evidence suggests that the activity of the toxic agent may be heightened by a possible lytic action of the blood serum of a sensitive individual, and that the sensitive serum itself may contain the toxic agent in solution. These results, as far as they go (omitting No. 10), accord with Langer's except on the point of alcoholic solubility and the effect of acids. The actual nature of the toxic agent in the black-fly is left a matter of speculation. The following working theories have suggested themselves to the writer. First, the toxin may be, as Langer believes in the case of the bee, an alkaloidal base, toxic as such, and neutralized after injection by antibodies produced for the occasion by the body. In such a case the view that a partial local fixation of the toxin occurs, which prevents its immediate diffusion, is acceptable. Through chemotactic action, special cells capable of breaking up the toxin into harmless elements are attracted to the scene. Their function may be, on the other hand, to neutralize directly, not by lysis. This would explain the rôle of the eosinophiles in the black-fly lesion. If their activities be essential to the destruction or neutralization of the toxin, one would expect them to be most numerous where there was least reaction. This would be at the site of a bite in an immune individual. A point of special interest for further investigation, would be the study of such a lesion. Second, it is conceivable that the injected saliva of the fly does not contain an agent toxic as such. It is possible, that like many foreign proteins, it only becomes toxic when broken down. The completeness and rapidity of the breaking down depends on the number of eosinophiles present. In such a case immunity should again be marked by intense eosinophilia. [Illustration: 173. Fifth day mature lesion. Lower power drawing showing papillary œdema and infiltrate in the region of the puncture. After Dr. J. H. Stokes]