Isolation of parietal cells from guinea-pig gastric mucosa was performed by density gradient centrifugation using percoll, a new gradient medium composed of colloidal silica coated with polyvinylpyrolidone. The gastric mucosa of the oxyntic gland area was exfoliated, minced and digested with 2.0 mg/ml of dispase and 0.1 mg/ml of collagenase. Dispersed mucosal cells were applied to the percoll density gradient. Parietal cells were fractionated in the layer between 1.04 and 1.06 g/ml in density with purity 80 approximately 90%. For identification of parietal cells staining with tetranitrobluetetrazolium and electron microscopy were used in addition to common staining. The viability of the cells was over 95% assessed by trypan-blue exclusion. The parietal cells obtained responded actively to histamine stimulation when 14C-aminopyrine uptake was used as an functional index. In conclusion, percoll density gradient medium is suitable for yielding isolated parietal cells with both high purity and high viability.