The objective of this study was to develop an in vitro technique to screen drugs for toxicity to laboratory cultured protozoans. Drugs were diluted using standard microtitration equipment. An easily cultured protozoan, Tetrahymena pyriformis, was added to known dilutions of drugs, alone and in combination. Drugs used as a model system to test the technique were formalin and malachite green. Toxic concentrations of formalin determined by this in vitro technique were close to recommended treatment levels. Malachite green was toxic at levels in excess of treatment concentrations. The synergistic effect of formalin and malachite green was demonstrated. This technique can be of use in determining effectiveness of new drugs as parasiticides.