A new model for obtaining the distributions of cells in the cell cycle by computer analysis of flow cytometry data is proposed. The model is able to analyze distributions from both asynchronous and synchronous cell populations. A culture of HeLa cells is pulse-labeled with tritiated thymidine at various times after synchronization by the mitotic selection method, then analyzed by flow cytometry and autoradiography. The results show good agreement between the percentage S-phase cells determined by the model and by autoradiography.