In the primate neocortex, little is known about the possible associations between functional subclasses of GABA neurons, their morphological properties and calcium-binding protein (CaBP) content. We used whole-cell current clamp recordings, combined with intracellular labeling and fluorescence immunohistochemistry, to determine these relationships for interneurons in layers 2-3 of monkey prefrontal cortex (PFC). Eighty-one interneurons were included in the analysis. Thirty-eight of these cells showed immunoreactivity for one of the three CaBPs tested. Co-localization of more than one CaBP was not observed in any of the interneurons examined. Interneurons with different CaBPs formed distinct populations with specific physiological membrane properties and morphological features. Parvalbumin (PV)-positive cells had the physiological properties characteristic of fast-spiking interneurons (FS) and the morphology of basket or chandelier neurons. Most calretinin (CR)-containing cells had the physiological properties ascribed to non-fast-spiking cells (non-FS) and a vertically oriented axonal morphology, similar to that of double bouquet cells. Calbindin (CB)-positive interneurons also had non-FS properties and included cells with double bouquet morphology or with a characteristic dense web of axonal collaterals in layer 1. Classification of the interneurons based on cluster analysis of multiple electrophysiological properties suggested the existence of at least two distinct groups of interneurons. The first group contained mainly PV-positive FS cells and the second group consisted predominantly of CR- and CB-positive non-FS interneurons. These findings may help to illuminate the functional roles of different groups of interneurons in primate PFC circuitry.