A simple in vitro method was developed to assess the toxic effect of experimental compounds, with two nonadhering cell lines, a parasitic protozoan, Leishmania mexicana (promastigote), and human lymphocyte CEM T4 cells. The technique developed involves growth of cells in a 96-well microwell plate and monitoring of turbidity. This technique requires the use of a chemically defined medium, nonadhering cells, and a microwell spectrophotometer. The method is rapid, simple, and can, in principle, be easily applied to determine the effect of test agents on any nonadhering cell line or microorganism.