SummarySignificantly drastic effects of storage on the potency of the alum‐precipitated haemorrhagic septicaemia (APHS) vaccine are reported. The APHS vaccine, studied through challenge infection of vaccinated rabbits (post‐60 days of vaccination), showed 100% potency when stored at 4 °C for 30 days. The potency dropped to 20% when storage period was extended to 60 or more days. At 30 °C, the potency reduced by 40, 40 and 60%, respectively, after 30, 60 and 90 days of storage, while, at 37 °C, the decrease was 60, 60 and 100% after 30, 60 and 90 days of storage, respectively. In view of this, the oil‐adjuvant (OA) HS vaccine was developed by culturingPasteurella multocidaon a medium comprising yeast extract, sucrose, trypticase and sodium bicarbonate, under continuous aeration at 37 °C. This gave a far better bacterial count (maximum count 15 times 108/ml) than the conventional APHS vaccine (maximum count 6 times 108/ml). The OAHS vaccine‐carrying water‐in‐oil emulsion remained stable at room temperature for 1 year. The log protection values of the two batches of the OAHS vaccine, studied in mice, were 5.2 and 5.3, as against 1.9 of t