A method of sealing the open mouths of an array of test tubes comprises providing a plurality of plugs with upwardly facing recesses, and a flexible carrier sheet having the same array of mutually spaced downwardly projecting pegs inserted into and frictionally retained within the recesses of respective ones of the plugs. Sealing is effected by aligning the plugs with the open mouths of the test tubes, pressing the carrier sheet towards the test tubes to seat the plugs in the open tube mouths, and peeling the carrier sheet away from the test tubes to extract the pegs from the plug recesses while allowing the plugs to remain seated in the tube mouths.