The volume measurement of macrophages from rabbit lungs was carried out using an automatic computerized cell counter. The size of freshly prepared macrophages was 669 ± 62 flat 4°C. The cell volume increased gradually when the cells were suspended in an isotonic saline solution. Cytochalasin B initially enhanced the increase in cell volume but then caused a reduction. The reduction was considered to be due to structural changes in filament network of contractile proteins. Under hypotonic conditions, cytochalasin B-treated cells showed increased osmotic fragility and reduced expansibility. The increased fragility was considered to result from impairment of the network structure. These findings of the effect of cytochalasin B on the volume change suggest that contractile proteins play a protective role in the membrane against hypotonic expansion.