Development of peripheral blood mononuclear cells (PBMC) responses to phytohemagglutinin (PHA) and concanavalin A (Con A) in 5 Japanese Black newborn calves was examined by the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) assay until the day 28 after birth. The PHA responses of PBMC from calves aged from 0 to 7 days were less than those of PBMC from control adult cattle. While the difference of the Con A response of PBMC between newborn calves and adults was not significant statistically. These results suggest that the response of T cell to PHA may be insufficient at early stage after birth in Japanese Black.