5-Lipoxygenase activating protein (FLAP) functions as a facilitator of 5-lipoxygenase (5-LOX) activity. However, on the basis of the induction of apoptosis by the FLAP inhibitor MK886 in cells lacking 5-LOX, it is possible that this fatty acid-binding protein has other activities. This study was designed to examine potential roles of FLAP in apoptosis and cell proliferation. Overexpression of FLAP protein (2.2-fold) was achieved by stable transfection of IL-3-dependent murine prolymphoid progenitor cells (FL5.12) with a construct expressing the cDNA under a CMV promoter. The overexpressed protein was localized to nuclear membranes as with endogenous FLAP. The initial growth rate of FLAP-transfected cells was greater than that of control cells. After 48 h, when cell density had increased, the growth rate of FLAP-transfected cells declined substantially and there and there was a decrease in viability relative to control transfected cells. The FLAP-transfected cells were also more susceptible to withdrawal of IL-3 than were control cells. There was, however, no difference between FLAP and control cells in their susceptibility to MK886, NDGA, or etoposide during the log growth phase. Overexpression of FLAP did not alter Bcl-xL protein expression, but did decrease Bax protein and somewhat increased COX-1 and COX-2 mRNA levels. The failure of increased FLAP to alter susceptibility to MK886 provides further support to the concept that this agent induces apoptosis by mechanisms unrelated to FLAP. The data also suggest that FLAP can affect cell proliferation.