An immunosuppressant. Selectively blocks signaling that leads to p70 S6 kinase activation (IC₅₀ = 50 pM). Inhibits lymphokine-induced cell proliferation at the G1 phase. Induces apoptosis in a murine B cell line. Rapamycin has been used in vivo studies by i.p., i.m., and i.v. administrations..