68 MEMBRANE PERMEABILITY AND DRUG ACTIOlSr 



ceptor grouping such as SH. Thus a trjrpanosome 

 is killed when a given proportion of its receptors is 

 saturated, i.e. when a given amount of arsenoxide^ 



TABLE XVI 



VALUES OF THE LETHAL DILUTION (l.D.) FOR 



the killing of trypanosomes by certain 



arsenoxides, and calculated values of 



(l.d.)M'/V5 



The relatively small variability of (L.D.)M^'^/B in- 

 dicates that most of the variation in L.D. is caused 

 by differences in the rates of permeation of the 

 arsenoxides into the trypanosome. 



irrespective of its structure, has penetrated into the 

 trypanosome. If this is true, the permeability of the 

 trypanosome must be the main factor determining the 

 value of the lethal dilution. Assuming that this is so, 

 and that the permeation group is group four, it is easy 



