A portable cell is described which allows X-ray diffraction data to be collected from suspensions or mounted specimens in liquid under hydrostatic pressure. Pressure is applied via either an oil pump or a high-performance liquid chromatography pump. The temperature of the cell is controlled by a thermostatted baseplate. X-rays pass through the stainless steel block via windows of Kapton V plastic sheet which will withstand a pressure difference of at least 15 MPa. Distortion of the plastic under pressure increases the liquid path of the X-rays. This distortion can be reduced by stretching over dome-shaped supports. Under these conditions 10 MPa causes a reduction of intensity of between 10 and 50%, depending on window geometry. Specimens can be mounted on a removeable frame, and the solution in the sample chamber can be changed without disassembling the cell. The apparatus has been used successfully to study the low-angle diffraction from muscle.