It was found by radioimmunoassay of serum calcitonin in rabbits that a hypocalcemic protein from bovine parotid gland lowers serum calcium without increasing the secretion of calcitonin. Binding of calcitonin and anti-calcitonin was not inhibited by the parotid protein. Furthermore, when calcitonin was administered to rabbits in addition to a maximally hypocalcemic dose of the parotid protein, it caused an additional decrease of blood calcium level. These results suggest that the target organs of the parotid protein and calcitonin are different, or that the parotid protein and calcitonin activate different receptors in the same target organ. Leucocyte-increasing activity was demonstrated in both the parotid protein and calcitonin, but the effect of calcitonin developed more rapidly than that of the parotid protein, as was the case with hypocalcemic activity. It was also found that the parotid protein caused a significant increase of rosette-forming cell activity in the spleen of neonatal mice, whereas calcitonin did not. These observations suggest that the biological effects of the parotid protein are not mediated by calcitonin.