Continued studies of the uptake and binding of antidepressant tricyclic drugs(imipramine and chlorimipramine) showed that the binding of chlorimipramine was competitively inhibited by imipramine and its analogs. Kinetic analyses of the equilibrium data revealed that binding of both drugs is saturable, and Scatchard plots indicated a single class of binding sites. The effect of pH on binding affinities showed increased sensitivity of the parasites with increasing pH values, probably owing to altered protonation of the peripheral membrane proteins, and the high pH values, probably owing to altered protonation of the peripheral membrane proteins, and the high pK value (8.3) of the drugs. Collaborative studies, initiated with Dr. Diamond this year, of the respiratory metabolism of Entamoeba species revealed a specific stimulatory effect of Min++. Distinct differences in molecular taxonomy also were found among these species.