Lysophosphatidic acid induced aggregation of human and feline platelets, the activity depending on the structure of the sn-1-acyl moiety. A hydroxyl group at the sn-2-position was not necessary for the activity, but a wedge shaped structure was suitable for induction of aggregation. Almost all the lysophospholipids tested with a head-group attached to the phosphate portion had little or no activity, but the sn-2-acetylated analogs of these phospholipids were very active. These phospholipids with activity were suggested to induce aggregation by interaction with the same binding site on the cell surface.