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Calculi that accumulated in 18 weeks on the teeth of cats were measured quantitatively by determining the ash weight and calcium content of specimens. Calculi accumulating in a control period were compared with that developing in the same cats in an experimental period, and the difference analyzed for statistical significance ( P = 0·05 or less). Erythromycin, penicillin, dihydrostreptomycin, and a combination of penicillin and dihydrostreptomycin produced significant or near significant reductions of calculi of 66, 43, 35, and 20 per cent, respectively. Tetracycline and chloramphenicol caused statistically insignificant decreases of 43 and 31 per cent, respectively. Saline caused an insignificant reduction of 8 per cent. Antibiotics (and saline) were administered daily intramuscularly and each tested on five cats. Ten cats with the parotid, submaxillary, and sublingual glands removed formed the same amount of calculi after the glands were removed as before. Ten sham-operated cats (controls) showed-an 11 per cent increase after the operation, but the increase was of doubtful significance. Teeth brushed either daily or twice-weekly on one side of the mouth had 95 per cent less calculi than unbrushed teeth on the opposite side. Teeth brushed once-weekly had 76 per cent less calculi than unbrushed teeth. Reductions from toothbrushing were all statistically significant.