Emergency services were called to the Mothers Pride bakery in Duke Street, Glasgow, at about 9pm last night.
The Scottish Ambulance Service said two casualties were taken to Glasgow Royal Infirmary.
One man was treated for head injuries and another was reported to have an arm injury. Two other men were treated for shock, the Scottish Fire and Rescue Service said.
Twenty firefighters from the local Calton, Cowcaddens and Polmadie Stations were at the scene of the explosion, along with police and ambulance services.
Firefighters were examining the bakery to ensure it was safe.
A spokesman for the the Scottish Fire and Rescue Service said: “At this stage there is no information as to the cause of the explosion.”