A method for controlling evaporation and contamination of reagents used in automated diagnostic systems is disclosed as well as an apparatus for opening and closing reagent containers contained within a reagent pack having cover and cap means for such controlling of evaporation and contamination. The method comprises (a) moving reagent packs containing reagents in closed reagent containers mounted in a reagent pack carousel to an opening and closing station, (b) forcing open the evaporatively closed reagent containers contained in the reagent pack by the opening and closing station, (c) locking in an open position the reagent containers cover and cap which is affixed to the reagent containers, (d) moving the locked open reagent containers from the opening and closing station on the reagent pack carousel to a position wherein the system has access to the reagents, (e) withdrawing reagents from the reagent containers for system use, (f) returning the reagent pack and reagent containers on the reagent pack carousel to the opening and closing station, (g) unlocking the lock open cover and cap of the reagent containers, (h) force closing the cover and cap of the reagent containers of the reagent pack, and (i) removing the reagent pack from the opening and closing station.