The ease of application of quinine prophylaxis, as compared with the more permanent methods of mosquito destruction and screening, appeals to the sanitarian, especially in the tropics. It is just as easy to give quinine to a man in the tropics as it is in temperate climates, but when one considers the propositions of draining tropical swamps and shutting off circulation of air on a torrid night with fine wire gauze in the windows and closely woven mosquito nets around the bed, the question is decidedly different. In consequence, the tendency is for the average man to despair of accomplishing anything in the way of mosquito destruction and screening and to seize eagerly on the inferior alternative, that of quinine prophylaxis. Ronald Ross presents this matter concisely and to the point when he states that it is not a good policy to substitute a measure which does not exclude infection, but is merely extirpative in some cases, for positive prevention. From this it will be seen that unless it is clearly recognized that quinine prophylaxis may in some cases extirpate, but does not prevent, there might be a tendency to adopt this measure and neglect the two proper ones. As regards the relative merits of quinine prophylaxis and protection from mosquitoes Celli gives the following figures: Treatment Infected Mosquito protection plus quinine prophylaxis 1.76 % Mosquito protection alone 2.5 % Quinine prophylaxis alone 20.0 % No protection at all 33.0 % With quinine prophylaxis, there is the possibility of producing an immunity to quinine on the part of the parasites which have been introduced by infected mosquitoes and held in check by the prophylactic but not curative dose of quinine. Later on when the quinine prophylaxis is discontinued the parasites begin to multiply vigorously and seem to possess an immunity to quinine. As an instance of this, 398 marines served in 1906 for about one month on the Isthmus of Panama during which time they were given 9 grains of quinine daily as a prophylactic. During this month there was only an occasional case of malaria among the men. At the end of the month 298 of the original 398 returned aboard ship and sailed for the North. Two days later 20 cases of malaria developed, followed the next day by 53 and the day following that by 45. The medical officer then resumed 10-grain prophylactic doses for those not down with malaria but notwithstanding this there were 215 acute malarial paroxysms, some of them of pernicious type, among the 298 men. It was noted that these men did not respond satisfactorily to quinine treatment even when the drug was administered intramuscularly.