A biological sample collection system includes (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 valve and including a reagent chamber for storing a sample preservation reagent. The valve is configured to retain the sample preservation reagent in the reagent chamber when the valve is in a closed configuration. The sealing cap is configured to associate and form a fluid tight connection with the sample collection vessel. Associating the sealing cap with the sample collection vessel causes the valve to be placed in an open configuration in which a fluid vent through the core is placed into fluid communication with the reagent chamber, thereby causing or allowing sample preservation reagent to pass from the regent chamber to the sample collection vessel.