A semi-automated colorimetric chemosensitivity assay was developed. The assay utilizes the vital stain neutral red for the rapid screening of potential anticancer agents using solid tumor cell lines. The cell lines used in this assay and presented in this report are CX-1 colon adenocarcinoma and A549 lung carcinoma. The assay, performed in 96 well tissue culture plates, allows for short drug exposure times (3 hrs.) followed by quantitation of cell number (neutral red absorbance) following four cell doubling times. Cell number directly correlated with absorbance of eluted neutral red at 540 nm. However, optimal amounts of dye and staining times varied between cell lines. IC50 concentrations (for inhibition of cell growth) determined using this assay were in good agreement with results from clonogenic assays using similar drug treatment conditions. The assay technique was determined to be capable of detecting antineoplastic compounds operating by a wide variety of mechanisms.