CD20 functions as a calcium-permeable cation channel. When expressed in Balb/c 3T3 cells, CD20 accelerates the G1 progression induced by insulin-like growth factor-I (IGF-I). To further characterize how CD20 modulates the action of IGF-I, we investigated whether the activity of CD20 channel was affected by IGF-I. In quiescent cells expressing CD20, IGF-I increased cytoplasmic free calcium concentration, [Ca2+]c, which was reversed by the removal of extracellular calcium. In contrast, IGF-I did not increase [Ca2+]c in cells that did not express CD20. In perforated patch clamp recordings, addition of IGF-I to the bath solution augmented the Ca2+ permeability, which was reversed by anti-CD20 antibody. In cell-attached patch, calcium-permeable channel activity with unitary conductance of 7 picosiemens was detected, which was abolished by anti-CD20 antibody. The single channel activities were markedly enhanced when IGF-I was included in the pipette solution, whereas IGF-I added to the bath solution was ineffective. When cells were first exposed to pertussis toxin, activation of the channel by IGF-I was blocked. Transfection of cDNA for Gip2, a constitutive active form of alphai2, activated the CD20 channel. These results indicate that the CD20 channel is regulated by the IGF-I receptor by a mechanism involving pertussis toxin-sensitive G protein.