Microturbidimetry has been used to measure the growth of cells in suspension. Disaggregated mouse NC carcinoma cells in culture medium were added to the wells of a microtitre test plate and incubated. Absorbance of 600 nm light was measured daily for 4 days using a microplate reader. As the cells grew, light absorbance increased. Methotrexate 2-40 ng ml-1 reduced cell growth; this effect was increased by indomethacin 1 microgram ml-1, possibly by displacing methotrexate from its binding by serum protein or by enhancing cell uptake of methotrexate. Similar results were obtained by conventional clonogenic assays. The new technique offers simplicity, better reproducibility, and substantial savings in time and cost.