With Reid's filtration technique as a bioassay for evaluating red blood cell (RBC) deformability, we found that tripalmitolein, triolein and trilinolein improved the deformability of calcium-loaded RBC while glycerol, linoleic acid, dilinoleoyl phosphatidyl choline, trilpalmitin, tristearin, trilinolenin and triarachidonin did not. Maximal effect of tripalmitolein, triolein and trilinolein was achieved at 10(-9), 10(-10) and 10(-8) M respectively. We propose that only those triacylglycerols which contain fatty acids with one or two double bonds could modify RBC membrane fluidity and improve RBC deformability.