The effect of Ca2+/calmodulin (CaM) on the specific binding of [I-125]omega-conotoxin GVIA (I-125-omega-CTX) to crude membranes from chick brain was investigated. When we examined the effects of the activation of various endogenous protein kinases on specific [I-125]omega-CTX binding to crude membranes, we observed that Ca2+/CaM had an inhibitory effect regardless of whether or not the standard medium contained ATP (0.5 mM). Ca2+/CaM also had an inhibitory effect in a simple binding-assay medium containing HEPES-HCl buffer, BSA, Ca2+ and CaM,and this effect was dependent on the concentration of Ca2+. The effect of Ca2+/CaM was attenuated by the CaM antagonists W-7 and CaM-kinase II fragment (290-309). An experiment with modified ELISA using purified anti omega-CTX antibody indicated that Ca2+/CaM did not affect the direct binding of [I-125]omega-CTX and CaM. These results suggest that Ca2+/CaM either directly or indirectly affects specific [I-125]omega-CTX binding sites, probably N-type Ca2+ channels in crude membranes from chick whole brain.