A separator for use in a container for a biological fluid is disclosed. The separator includes a separator body having a first region at least partially made of a first material having a first density, and a second region at least partially made of a second material having a second density different from the first density. The separator has an overall density intermediate the densities of a first phase and a second phase of the biological fluid. At least a portion of the separator is deformable between a first condition in which the biological fluid can pass between an inner surface of the container and the separator, and a second condition in which at least a portion of the separator prevents the biological fluid from passing between the inner surface of the container and the separator.