We have previously reported that the number of metastases resulting from the inoculation of TA3 ascites tumor cells into mice could be reduced by 50 per cent if the mice were first injected with neuraminidase.'It was assumed that this effect probably reflected the action of neuraminidase on the sialic acid coat of tumor or host endothelial cells, since it was believed that mutual stickiness be-tween these cell types was important in metastasis formation.'Since the uncoating of tumor cells by neuraminidase prior to injection into the host seemed to have very little antimetastatic effect,'attention was directed to the effects of neuraminidase on the host. The present paper reports the unexpected finding that neuraminidase in vivo produces thrombocytopenia, with a close correspondence between the decreasein platelet levels and the decrease in metastases. Further experiments, in which the concentration of circulating platelets was varied by other agents, confirm the association between platelet levels and metastases, and suggest that the previously observed antimetastatic effect of neuraminidase may be at least partially mediated by its effect on the platelets.

Material8 and Methods.-TA3 ascites tumor cells2 were grown in the peritoneal cavity of CAFi/Jax female mice, each receiving 2.0-2.5 X 105 or 3.0-3.5 X 105 tumor cells sus-pended in 0.2 ml of medium. They were killed 14-16 days later, and ether and lung metastases were counted.