Macroscopic and single-channel properties of a cloned human heart L-type Ca2+ channel alpha 1 subunit expressed in Xenopus oocytes were studied and the effects of a cardiac beta subunit were evaluated. The alpha 1 subunit expressed current with much slower activation and inactivation kinetics than native cells. The beta subunit increased the current amplitude and accelerated both activation and inactivation rates. Single-channel analysis revealed that the beta subunit increased the probability of channel opening and shifted the voltage-dependence to more negative potentials without affecting conductance or open time. The data suggest that the beta subunit modulates macroscopic current by increasing the probability of channel opening and shifting the voltage-dependence of the channel.