Immunoreactivity for two calcium binding proteins, 28 kDa calbindin and parvalbumin, was used to label cells morphologically identical to Cajal-Retzius neurons in the developing visual, prefrontal, sensory-motor and temporal cortex of Old World monkeys. At all fetal ages examined (E110-E155), Cajal-Retzius neurons throughout the cortex were immunoreactive for calbindin as well as being acetylcholinesterase positive. Between E130 and E150, the calbindin-immunoreactive Cajal-Retzius cells in the visual cortex, and a few in other cortical areas, also showed parvalbumin immunoreactivity. A reduced population of immunoreactive Cajal-Retzius cells was detected at birth, and none could be visualized by immunocytochemistry or histochemistry at later postnatal ages. Calbindin and parvalbumin immunoreactivity represents a potentially useful marker for this developmentally regulated population of neurons, and the varied expression of the two proteins suggests that Cajal-Retzius neurons may represent a neurochemically heterogeneous cell population.