The severe limitation of the capacity of working memory, the ability to store temporarily and manipulate information, necessitates mechanisms that restrict access to it. Here we report tests to discover whether the activity of neurons in the prefrontal (PF) cortex, the putative neural correlate of working memory, might reflect these mechanisms and preferentially represent behaviourally relevant information. Monkeys performed a 'delayed-matching-to-sample' task with an array of three objects. Only one of the objects in the array was relevant for task performance and the monkeys needed to find that object (the target) and remember its location. For many PF neurons, activity to physically identical arrays varied with the target location; the location of the non-target objects had little or no influence on activity. Information about the target location was present in activity as early as 140ms after array onset. Also, information about which object was the target was reflected in the sustained activity of many PF neurons. These results suggest that the prefrontal cortex is involved in selecting and maintaining behaviourally relevant information.