A bstractThe acrosom e reaction is a Ca 2+ -dependent exocytotic process that is a prerequisite step for fertilization. External calcium entry through voltage-activated Ca 2+ channels is know n to be essential in inducing the acrosome reaction of mammalian sperm atozoa. Due to their com plex geom etry, however, electrophysiological identification of sperm Ca 2+ channels has been lim ited. Here we identified Ca 2+ channel m RNAs expressed in m otile hum an sperm using RT-PCR and their levels w ere compared using RNase protection assays. L-type, non-L-type, and T-type C a 2+ channel m RNAs w ere detected by RT-PCR using degenerate prim ers. Cloning and sequencing of the PCR products revealed α1B, α1C, α1E, α1G , and α1H sequences. RT-PCR using specific prim ers repeatedly detected α1B, α1C, α1E, α1G , and α1H m RNAs, and additionally α1I m RNA. But α1A and α1D m essages w ere not detected. Relative expression levels of the detected Ca 2+ channel subtypes were compared by RNase protection assays. The abundance of detected m RNA m essages w as in the follow ing order: α1H α1G α1E α1B α1C α1I. These findings indicated that hum an m otile sperm express m ultiple voltage-activated Ca 2+ channel RNAs am ong which T-type and non-L-type channel m essages are likely to be predom inantly expressed. Based on their relative expression levels, w e propose that not only T-type but also non-L-type calcium channels m ay be m ajor gates for the external calcium influx, required for the acrosom e reaction.