The influence of IPD-1151T (suplatast-tosilate) on murine mast cell induction was examined by in vitro cell culture. Splenocytes from BALB/c mice suspended in RPMI-1640 medium supplemented with interleukin-3 were cultured in the presence or absence of IPD-1151T. Half of the total volume of the medium was changed every 4-5 days. The number of mast cells increased with culture time and reached a peak on the 16th day when the cells were cultured without IPD-1151T. However, the growth of mast cells from the splenocyte culture was dose dependently inhibited by IPD-1151T. Furthermore, IPD-1151T inhibited the proliferation of mast cell progenitor cells, IC-2, but not that of splenocytes or mature mast cells.