a reuseable plastic cap is disposed over a seal closure on a vial neck to retain the seal closure in place thereon , the cap comprising a body of resilient material having a base overlying the seal closure , opposed jaws on the base for gripping a bead on the vial neck , gripper flanges for manual engagement to spread apart the jaws so as to install or remove the cap on the vial neck , and a releasable locking bar to hold the gripper flanges spread apart and thereby retain the jaws in a locked condition on the vial neck .