During a collection interval, if a multi-threaded generational, copying garbage collector finds that a card contains a reference to a younger generation, it gives a corresponding card table entry a youngergen value selected from a plurality of youngergen values at the beginning of the interval as the "current" youngergen value. The youngergen value chosen for a given collection interval differs from the one that was chosen for the previous collection interval. As a result, a collector thread is able to distinguish between a card in which a reference to the younger generation remained at the end of the previous interval and one identified by some other thread during the current interval as referring to a young-generation object. In that way, the thread is able to avoid unnecessary scanning.