The calcium ionophore properties of A23187 and of two lysophosphatidic acid (LPA) analogs (1-palmitoyl- and 1-hexadecyl-sn-glycero-3-phosphate or P-GPA and H-GPA, respectively) were compared using platelet membrane vesicles loaded with 45Ca. Half maximal effect (HME) was obtained at 5 microM and 10 microM for H-GPA and P-GPA, respectively, against 0.7 microM for A23187, which released 2 times more Ca. The three compounds also induced platelet aggregation with a HME at 0.5 microM, 0.3 microM and 0.01 microM for A23187, P-GPA and H-GPA, respectively. The clear dissociation between the two effects appearing for both LPA raises some doubt about the general idea that (lyso) PA participate in cell activation through their calcium ionophore properties.