Calmodulin, a highly conserved protein family that has long been well known as an intracellular calcium sensor, was identified in the culture medium and cell walls of Arabidopsis thaliana suspension-cultured cells by immunoblotting assay. A promotion effect by applying exogenous purified calmodulin and an inhibition effect by the addition of anti-calmodulin anti-serum or calmodulin antagonist to the medium on proliferation of suspension cells were found by monitoring incorporation of [methyl-3H]thymidine into nuclear DNA. Radioligand binding analysis with 35S-labeled calmodulin indicated the presence of specific, reversible, and saturable calmodulin binding sites on the surface of both A. thaliana suspension-cultured cells and its protoplasts; among them at least one is on the surface of Arabidopsis protoplasts, with the Kd approximately 9.2 nM, and two are on the out-surface of Arabidopsis suspension-cultured cells, with Kd values of approximately 47.5 and 830 nM. Chemical crosslinking of 35S-labeled calmodulin to protoplasts revealed 117- and 41-kDa plasma membrane proteins specifically bound to calmodulin, whereas cross-linking with intact suspension-cultured cells verified more calmodulin binding proteins which might be cell wall-associated in addition to membrane-localized. Taking together, our data provide first evidence for the presence of apoplastic calmodulin receptor-like binding proteins on the cell surface of Arabidopsis suspension-cultured cells, which strongly supports our previous idea that apoplastic calmodulin functions as a peptide signal involved in regulation of cell growth and development.