Serum sickness is a delayed immune reaction in which the immune system responds to a protein in antiserum as a potentially harmful substance and mounts an IgG-mediated antibody response. A 32 year-old female patient had systemic envenoming following a bite by a red-bellied black snake (Pseudechis porphyriacus). She was treated with Tiger snake antivenom and recovered over 24 h and did not develop myotoxicity. She then presented with local pain, itching and swelling, which was partially treated with antihistamines. Eleven days after the bite she presented again with symptoms of worsening serum sickness including rash on the upper legs, joint and muscle pain in arms, ankles and knees, and nausea. The patient was prescribed five days of prednisone 50 mg/day, antihistamine 10 mg/day and analgesia 1000 mg/day and improved over 2 days. She had no further problems on follow up at 4 months. This case highlights that serum sickness can cause significant effects after the treatment of snake envenoming. It develops 5-14 days after antivenom administration and has characteristic clinical and laboratory features. Severe cases of serum sickness can result in morbidity but it appears to respond well to corticosteroid treatment.