In the monolayer culture of murine fibroblasts, the characterization of cellular motions was performed by morphological observation using a tool consisting of an optical assembly connected to a computer-aided image analysis system. This tool enabled the estimation of the projected area of the cells with sufficient accuracy, being able to follow the serial behavior of the cells on the culture surface. Trypsinization was chosen as an external factor for altering the state of the cells, and the rate of cell spreading (r(S)) and time of first cell division (t(1)) were evaluated by subjecting the cells to trypsin treatment for t(T)=3 and 15 min. During culture of the cells treated for t(T)=3 min, the r(S) values of individual cells exhibited a broad variation, ranging from 41 to 321 mum(2)/h, and the average r(S) was 165+/-78 mum(2)/h, which was 1.5 times larger than that of the cells treated for t(T)=15 min. On the contrary, the average t(1) of the cells treated for t(T)=3 min was 9.5 h which was 60% reduced as compared with that of the cells treated for t(T)=15 min. The prolonged time of trypsin treatment was considered to induce the decrease in r(S) and delay of the first cell division due to the requirement for the recovery from cell surface damage caused by trypsinization. The logarithmic plots of r(S) and t(1) were found to have an inverse relation regardless of the state of the individual cells.