Hybridoma lines producing anti-idiotypic monoclonal antibodies (AImAbs) were prepared by fusing splenocytes of mice immunized with α-latrotoxin (LT) and P3-X63Ag8.653 myeloma line cells. AImAbs (1) bind to the rat brain synaptosomes, (2) do not affect the LT binding to the high-affinity receptor, and (3) do not react with LT in solution. The effect of AImAbs on the LT-induced45Ca2+influx into rat brain synaptosomes was studied. Some antibodies (6.6D11 and 11.7B7) were found to strongly inhibit this process. The results obtained indicate that the presynaptic membrane contains unidentified components interacting with LT. The distortion of the interaction of LT with these unknown components affects the LT-induced calcium influx into synaptosomes.