Normal and tumor-bearing (plasma cell tumor-70429) mice were experimentally infected with the lactic dehydrogenase elevating agent. Within 72 hours after injection of this agent into normal mice the plasma lactic dehydrogenase (PLDH) activity increased from a normal base line of 500 units to 2000 units. The PLDH of the uninfected tumor-bearing animals gradually rose over the course of 17 days to 5,700 units, while the PLDH of the tumor-bearing animals infected with the lactic dehydrogenase agent rose to 44,000 units. The data indicate that the early rise in PLDH following tumor implantation is due to the lactic dehydrogenase elevating agent rather than the tumor. The uninfected tumor-bearing animals showed a gradual increase in PLDH which paralleled the gross increase in tumor size, but this increase was small when compared to the PLDH activity of the infected tumor-bearing animals. In light of the reported association of this agent with a wide variety of serially transplanted mouse tumors, the present data suggest that the lactic dehydrogenase agent may contribute to the increase in the plasma enzyme activity of many tumor-bearing mice. Studies with other enzymes are reported.