A preliminary study of blebbing in tissue cultures has been made. The tubal epithelium of fetal mouse oviduct was cultured at 37 degrees C in Rose chambers. A cinematographic record was obtained of phase microscope observations of mitotic cells. Measurements of the size of both cells and blebs were made on the film using a "traveling" microscope. The duration and the rise and decay times of blebs were determined simply by counting frames on the film. Detailed observations are reported on blebbing in four cells undergoing mitosis. The results indicate that the frequency of blebbing as well as the duration of individual blebs exhibits a maximum during telophase. A model is proposed to account for blebbing in mitotic cells. The model attributes to local regions of the cell membrane the property of constant tension independent of stretch over some restricted range of stretch. This property implies that the cell membrane is locally unstable. Further assumptions stated explicitly in the model are that (i) cell division occurs at constant volume, (ii) the cell membrane stretches during cleavage, (iii) there is a positive pressure drop across the cell membrane. Evidence is cited in support of these assumptions as well as independent evidence that the cell membrane may be locally unstable. A physical model is described which would be expected to exhibit blebbing given the above assumptions.