Comparison of published methods for the quantification of adherent cell numbers by the measurement of absorbance of bound stain indicates a wide variation in their sensitivity. This study aimed at comparing the sensitivities of five different staining procedures (Coomassie brilliant blue G in perchloric acid, Coomassie brilliant blue G in phosphoric acid, methylene blue, crystal violet, and toluidine blue) applied to three separate types of cultured fibroblasts (3T3 cells, Vero cells, and human gingival fibroblasts) at concentrations from 0.125 x 10(4) to 10 x 10(4) per well in 96-well microplates. Absorbance values of Coomassie blue-stained cells were measured in situ. Those of the remaining cells were measured after solubilization of the dye with 1% sodium dodecyl sulfate. All absorbance values were measured using an Elisa reader at 620 or 570 nm for crystal violet. The relationship between cell number and absorbance over the entire cell concentration range was best fitted with quadratic regression analysis, in contrast with the linear relationship described elsewhere. The order of sensitivity of the staining procedures was the same for each cell type: Coomassie blue in perchloric acid less than Coomassie blue in phosphoric acid less than methylene blue less than crystal violet less than toluidine blue. With the latter two stains absorbance values began to plateau at approximately 8 x 10(4) cells per well. However, staining with Coomassie blue in perchloric acid and methylene blue resulted in an almost linear relationship between cell number and absorbance over the entire concentration range tested.(ABSTRACT TRUNCATED AT 250 WORDS)