The murine cell line C3HA has been used extensively in studies of the cytopathology that accompanies TNF-induced cytolysis. This cell line undergoes an apoptic form of cell death characterized by plasma membrane blebbing and cytoplasmic boiling. Since plasma membrane blebs also appear on C3HA cells during mitosis, in this report we have compared these blebs with those that appear during apoptosis to determine whether they represent related structures. Our results reveal several differences. During mitosis, the blebs that appear are smaller and more heterogeneous in size than are those that appear during apoptosis. In addition, during mitosis bleb formation is preceded by the appearance of microvilli on the cell surface. No microvilli are observed during apoptosis. The staining pattern with rhodamine phalloidin also differed between mitotic and apoptic blebs, indicating a difference in their content of f-actin. The blebs that form during mitosis stained in a bright, uniform manner, suggesting an association with f-actin. In contrast, apoptic blebs were stained only at their base, the bleb itself being devoid of f-actin-associated staining. This difference may help explain why mitotic blebs are reintegrated into the cell surface, while the blebs that form during apoptosis are not.