We have studied the action of an alpha-latrotoxin (alpha-LTX) complex of two polypeptides (LTX 130 kDa and low molecular weight protein (LMWP) 8 kDa) and the action of a venom fraction containing LTX with excess LMWP on calcium influx into synaptosomes and PC12 cells as well as on [14C]GABA release from synaptosomes. Both preparations considerably activate calcium influx and stimulate [14C]GABA release from synaptosomes. Preincubation of both preparations with antibodies against a 14 amino acid residue C-terminal peptide of LMWP differentially modulates these effects. Antibodies inhibit induced calcium influx and enhance induced GABA release.