A biological sample collection system can include (i) a sample collection vessel having an opening for receiving a biological sample, (ii) a selectively movable valve comprising a core and a collar disposed about the core, and (iii) a sealing cap coupled to the collar and comprising a reagent chamber for storing a measure of sample preservation reagent. The sealing cap is configured to associate—and form a fluid tight connection—with the sample collection vessel such that associating the sealing cap with the sample collection vessel causes a physical rearrangement of the core relative to the collar such that a fluid vent associated with the core is moved into fluid communication with the reagent chamber, thereby permitting sample preservation reagent to pass from the regent chamber to the sample collection vessel.