The Fire Department responded to a call from what they thought was a frozen food factory in Pontian, Johor.
The fire had broken out at the back of the two-storey building in an industrial estate opposite the former immigration detention centre.
The firemen acted quickly to put out the fire, the New Straits Times reported.
But when they entered the building, they didn't find any frozen food, or any indication that the factory was used to process food.
Instead, the firemen stumbled upon what is believed to be a drug laboratory.
Johor police chief Mohd Mokhtar Mohd Shariff said the value of drugs being processed at the lab was about RM10 million (S$4 million).
Johor Fire and Rescue Department assistant director of operations Hamid Suari said they received a distress call concerning the premises at about 1.30pm on Monday.