In vivo and in vitro, keratinocyte differentiation is linked with increased extracellular Ca2+. In order to correlate ion channels with cell differentiation and investigate keratinocyte membrane responses to Ca2+, keratinocyte single channel currents were studied using the patch-clamp technique. The most frequently observed channel was a 14 pS nonspecific cation channel. This channel was permeable to Ca2+ and activated by physiological concentrations of Ca2+. We also found a 35 pS Cl- channel whose open probability increased with depolarization. Finally, a 70 pS K+ channel was seen only in cell-attached or nystatin-permeabilized patches. We correlated channel types with staining for involucrin, an early marker of keratinocyte differentiation. While the nonspecific cation channel and Cl- channel were seen in both involucrin positive and involucrin negative cells, all channels in which the K+ channel activity was present were involucrin positive. Membrane currents through these channels may be one pathway by which signals for keratinocyte proliferation or differentiation are sent.