A fibroblast-like BHK-21 (C-13) line, grown as monolayer on glass cover-slips, was incubated for the demonstration of acid phosphatase by the lead capture method. The density of final reaction product was measured with a scanning microdensitometer. A peak absorption was found at 510 nm. The extinction of discrete areas of stain was within the range of linear response of the instrument. The frequency distribution of the density of reaction product in 100 cells was normal (Gaussian), and the density increased linearly with incubation time. It is concluded that this method is valid for the measurement of the density of lead sulphide reaction product in cultured cells.