SUMMARY: Acriflavine-induced photosensitivity has been used to measure resistance in trypanosomes to certain aromatic arsenical drugs. Trypanosomes were incubated in a logarithmic dilution series of acriflavine and slides were prepared. After a standard exposure to light the % mortality as judged by mobility was calculated and converted into probits. Mean sensitivity and its standard deviation were estimated from graphs of probits log concentration of acriflavine. It is possible by a graphical method of analysis to distinguish between two or three populations even when these occur in the same infection. Numerical estimates of replicate analyses and analyses made on the same strain over a period of 2 1/2 years show that the system of measurement is stable. A range of 0.12 log units has been found in all measurements made over the period on the normal strain of This may be compared with the lowest degree of stable resistance which is an increase of log unit from the normal strain. Some factors which affect photosensitivity were examined, including temperature, action spectrum, concentration of drug, concentration of trypanosomes, time of incubation and the effect of oxygen. The method described is thought to be a compromise between that which is theoretically desirable and that which is simple to perform while still having an accuracy necessary for detailed studies of the development of drug resistance and of inheritance in trypanosomes.


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