C(60)-Fullerene trisamine adducts inhibit neuronal nitric oxide synthase and calcineurin phosphatase activities in a manner completely reversible by calmodulin. As measured by difference spectroscopy, D(3)-trisamine and C(3)-semiamine fullerene adducts displace trifluoperazine bound to calmodulin coincident with their binding. These binding events are complete at a molar ratio of 4 mol added fullerene per mole calmodulin. Trisamine fullerene adducts alter the native electrophoretic mobility of calmodulin, producing a heterogeneity of bands with associated fullerene. D(3)- and C(3)-trisamine fullerene adducts interact with dansylated calmodulin, producing a 50% loss of maximal fluorescence at concentrations of 30 nM. At higher concentrations than those required to inhibit neuronal nitric oxide synthase, trisamine fullerene adducts inhibit nitric oxide formation by the cytokine-inducible nitric oxide synthase isoform. These inhibitions are fully reversible by calmodulin and skeletal muscle troponin C but not by skeletal muscle parvalbumin. Of the trisamine fullerene adducts tested only the C(3)- and D(3)-semiamine adducts inhibit Ca(2+)-dependent nitric oxide production in GH(3) pituitary cells. These observations support the proposal that trisamine C(60)-fullerene adducts are potent calmodulin antagonists, some of which display activity in intact cellular systems.