A fluid-dispenser pump having a pump body (10) containing a measuring chamber (20) defined between an upper piston (30) sliding in an upper portion (11) of the pump body (10), and a lower piston (40) sliding in a lower portion (12) of the pump body (10). The lower piston (40) co-operates with a spring (50) disposed in the lower portion (12) of the pump body (10). The spring (50) is isolated from any contact with the fluid, and the lower portion (12) of the pump body includes an air-intake (15) enabling air to be admitted and/or expelled during actuation of the pump. Isolation is provided to prevent fluid from penetrating into the air-intake (15).