A device for the collection of oral fluid from a subject's oral cavity may include a sample-collecting mouthpiece and a one-way valve. The mouthpiece may include a collecting chamber, introducible into the oral cavity of the subject for collecting oral fluid, and a resiliently deformable wall section defining at least part of the collecting chamber such that compression of the resiliently deformable wall forcibly expels a collected fluid from the collecting chamber via a collecting chamber outlet. The one-way valve may be in fluid communication with the collecting chamber outlet and may allow fluid to be displaced from the device following compression of the resiliently deformable wall but prevents ambient air flowing back through the valve into the device.