A clinostat for simulating microgravity on cell systems carried in a fiber fixedly mounted in a rotatable culture vessel is disclosed. The clinostat is rotated horizontally along its longitudinal axis to simulate microgravity or vertically as a control response. Cells are injected into the fiber and the ends of the fiber are sealed and secured to spaced end pieces of a fiber holder assembly which consists of the end pieces, a hollow fiber, a culture vessel, and a tension spring with three alignment pins. The tension spring is positioned around the culture vessel with its ends abutting the end pieces for alignment of the spring. After the fiber is secured, the spring is decompressed to maintain tension on the fiber while it is being rotated. This assures that the fiber remains aligned along the axis of rotation. The fiber assembly is placed in the culture vessel and culture medium is added. The culture vessel is then inserted into the rotatable portion of the clinostat and subjected to rotate at selected rpms. The internal diameter of the hollow fiber determines the distance the cells are from the axis of rotation
