Voltage clamp studies of macrophages from cultures of mouse spleen macrophages produced N-shaped steady-state current-voltage curves containing a region of negative slope resistance. Some macrophages exhibit two stable states of membrane potential, having current-voltage relationships that cross the voltage axis at three points. Outward currents that turn on at voltages of +15 millivolts or greater were noted in several cells. The addition of barium chloride to the bathing medium abolished the negative slope resistance and reduced the inward currents in response to hyperpolarizing voltage steps. These data provide direct evidence that macrophages exhibit at least tow different voltage-dependent conductances and demonstrate that voltage clamp techniques can be useful in studying the membrane properties of leukocytes.