PtdIns liposomes, at a concentration of 40 μm, induced in FLF the synthesis of t-PA-Ag, and enhanced 45Ca2+ uptake. The induction of t-PA-Ag biosynthesis by PtdIns liposomes in FLF was inhibited by 5-15 μm verapamil, an inhibitor of Ca2+ uptake via the so-called "slow channels" by 0.5-10 μm TFP, an inhibitor of Ca2+ transport ATPase, and by 10-90 μm TMB-8, an inhibitor of intracellular Ca2+ mobilization. t-PA-Ag secretion was inhibited by decreasing the Ca2+ concentration <1.2 mm. On the other hand, addition of 0.08 μm of calcium ionophore A23187 increased t-PA-Ag biosynthesis after 72 hr of incubation by 247% (P < 0.01). These data support previous results and indicate that the synthesis of t-PA in FLF is Ca2+ dependent. Thus, it is suggested that PtdIns liposomes increase t-PA biosynthesis by affecting calcium metabolism.