Calmodulin (CaM) is the primary mediator of Ca signal transduction processes in cells. Unlike animal cells, plant cells express multiple CaM isoforms. One cloned soybean CaM isoform (SCaM-4) half-maximally activated mammalian nitric oxide synthase (NOS) at 180 nM while another (SCaM-1) served as a competitive antagonist (Ki approximately 120 nM) of this activation. The reciprocal was true for the protein phosphatase calcineurin (CaN); SCaM-1 half-maximally activated mammalian CaN at approximately 12 nM, and SCaM-4 competitively antagonized (Ki approximately 70 nM) its activation. The reciprocal enzyme activation and competitive inhibition exhibited by these plant CaM isoforms suggest that their differential expression in cells could allow selective activation of some target enzymes and the selective inhibition of others. This may allow for a branching or bifurcation in the Ca2+-CaM signal transduction pathway and to alterations in cell function.