Abstract: A sample selecting, positioning and retrieving mechanism embodies a carousel having preferably inner and outer concentric rows of cylindrical tubes placed vertically between parallel upper and lower carousel plates. Each vertical tube is equipped with a retaining catch to maintain a sample in its resting position until it is released into a probe situated in a polarizing magnetic field and, conversely, to hold it in the rest position after it has been returned from the probe. A detent mechanism sequentially advances the carousel, one tube at a time, until all of the samples in one of the rows have been processed. A movable shifter plate, on which the carousel is journalled, repositions the carousel such that the other sample row is in alignment with the magnet air column. The mechanism is preferably enabled for movement by pneumatic cylinders and is capable of manual or unattended, programmed operation under computer control.