A simple, flexible, low‐cost image analyzer system is presented for determination of the total number of cells and the number of viable cells in monolayers of nonconfluent cells. Viable cells were identified and counted using an immunofluorescence method using fluorescein diacetate. Alveolar macrophages from rabbits were used. The different possible counting procedures that could be performed are analyzed. The precision of the method of estimating the cell numbers is indicated by a coefficient of variation of 5%. The experimental procedure is given for a short‐term toxicological in vitro test in which the semiautomatic counting procedures can be performed within 1.5 h.