This study examined the ability of anti-saxitoxin rabbit serum to neutralize saxitoxin, both in vitro and in vivo. In vitro, two rabbit antisera decreased [3H]-saxitoxin binding to specific sites in rat brain membranes. The more potent of these sera, antiserum A, when combined with saxitoxin in vitro, decreased saxitoxin's lethal potency based on mouse bioassay. Antiserum A also neutralized saxitoxin in vivo, as illustrated by the fact that mice injected i.p. with antiserum A (1:4) survived a s.c. injection 1 hr later of 16.7 micrograms saxitoxin/kg (1 LD99). Finally, antiserum A prevented death when injected i.v. immediately after s.c. injection of 16.7 micrograms saxitoxin/kg, however, antiserum injected by the i.p. and i.m. routes caused no significant increase in survival. This study indicates that antiserum can neutralize saxitoxin both in vitro and in vivo.