The ultimate objective of this project is to develop new methodologies to measure and distinguish between cytotoxicity and growth inhibition by toxic chemicals, carcinogenic agents and chemotherapeutic drugs. The specific aims include: (a) To establish the feasibility of measuring cell numbers in multiwell microculture plates by image analysis. (b) To validate the image analysis approach for (i) cytotoxicity testing using short term exposure to selected toxic substances, and (ii) growth inhibition at low cell density and continuous long-term exposure. (c) To apply computer-assisted acquisition and analysis of the image analysis data. The technical innovation in this project lies in the application of existing image analysis technology to the problem of direct measurement of cell numbers in situ in microwell culture dishes, thereby avoiding time-consuming, labor-intensive procedures designed to measure cell number by counting dispersed cell suspensions or by biochemical methods.