AIMS There are no studies measuring antivenom concentrations following intramuscular administration. This study aimed to compare antivenom concentrations following intravenous and intramuscular administration of redback spider antivenom (RBSAV).METHODS Twenty patients recruited to a controlled trial comparing intramuscular and intravenous administration of antivenom had serial blood samples collected at 30 min intervals for 2 h after the administration of one or two doses of antivenom. Antivenom concentration was measured using an enzyme immunoassay.RESULTS Ten patients received intramuscular antivenom but antivenom could not be detected in serum after either one or two vials, at any time point. The median time of the final sample after commencement of antivenom treatment in these patients was 3.2 h (1.8 - 5 h). Ten patients received intravenous antivenom (three one vial and seven two or more vials) and antivenom was detected in all patients.CONCLUSIONS RBS AV given by the intramuscular route is unlikely to be effective in the treatment of redback (widow) spider bite.