An analytical method for the measurement of cell growth in tissue culture is described, based on the Lowry method for the determination of protein, and employing a phenol reagent (Folin-Ciocalteau) for color development. The results, referred to a bovine serum albumin standard, may be converted to dry weight, nitrogen or cell count by appropriate conversion factors. Those factors are here given for 7 human cell lines and 1 mouse line.