By using an image‐analyzer system, toxicological effects of different substances on nonconfluent cells in monolayers can be assessed. The total number of cells and the number of viable cells are semiautomatically counted. Fluorescein diacetate is used as the viability marker. The method was tested by using monolayers of alveolar macrophages from rabbits exposed to manganese dioxide particles. A dose‐response curve was obtained from a series of experiments in which the response at a certain dose level can be obtained from cell‐counting procedures of the duration of 1 h. The method can be used as a short‐term test, testing potentially toxic substances including cytostatic drugs where the viability should be determined. Another example is further given of the use of this technique. The results obtained from it are compared with estimates of the percentage of viable cells assessed by extracellular lactate dehydrogenase from nonexposed and manganese dioxide particle‐exposed monolayers of rabbit alveolar macrophages. The two methods both account well for the theoretical relation when the lactate dehydrogenase activity values are corrected for an inactivation of this enzyme with time.