A method for enumerating G cells in dog antral mucosa by a fluorescent antibody technique was shown to be valid with respect to specificity and reproducibility. The state of intracellular gastrin storage does not influence G cell identification by this technique. To obtain values representative of the entire antrum, all G cells in long strips of antral mucosa must be counted. Results are best expressed as G cells per centimeter length of mucosa. Even with large mucosal samples there are considerable variations in G cell density within a single antrum. The "average" dog antrum contains about 35 million G cells. We observed as much as 2-fold range in G cell density from one dog to another. Each animal should, therefore, serve as its own control when influences on G cell populations are to be studied experimentally.