Abstract: To sample liquid phase components of a biological fluid which has been subjected to centrifugation to separate its liquid and semisolid phase components and define a liquid-semisolid interface therebetween, a probe carrying a sensor at its end is extended into and through the liquid phase components until the sensor detects the interface, whereupon further extension of the probe into the fluid is halted. A sample of liquid phase components is then withdrawn through openings in the probe spaced from the interface, so that only liquid components are sampled. It is contemplated that a stopper close a vessel containing the biological fluid, and that the probe be extended through and movable within a needle which first pierces the stopper, so that the probe may be moved through the needle and into the vessel. It is also contemplated that the withdrawn liquid phase components be aseptically transferred to a receiving vessel, and that the probe be cleansed with a flushing solution between sampling operations.