The number of circulating leucocytes from heart blood samples of oysters and the heart rate of oysters increase linearly with the rising ambient temperature at which the oysters were held. At ambient temperatures of 6[degree], 12[degree], 18[degree] and 22[degree] C, the mean leucocyte number per ml. heart blood was found to be 1.6, 2.7, 4.1 and 4.9 million, respectively. The corresponding heart rates were 5 (10[degree] C), 9, 19, and 26 beats per minute, These findings indicate that the fluctuation of leucocyte counts in the blood of experimental oysters is probably associated with the intensity of agitation exerted by the heart beat, which is in turn influenced by the temperature. Effects of repeated bleedings and of intraventricular injections of sea water and spinach chloroplasts on heart rate and number of leucocytes were also studied. Repeated bleedings do not affect the heart rate and the number of leucocytes, while injection of sea water and spinach chloroplasts does reduce the heart rate and the leucocyte number significantly for a period of 2 to at least 4 hours, depending on the type of inoculum used. These effects, therefore, do influence (1) the settling or sludging of leucocytes, (2) the mixing time of the inoculum and (3) the probable accumulation of leucocytes to the site of invasion, especially in oysters at lower temperatures.