The importance of auxin as a signalling substance with hormonal character is well documented. Apart from various auxin-binding proteins described by several groups, little is known of downstream elements in the signal cascade triggered by auxin. A phospholipase A2 activated by auxin in vivo and in vitro was recently described by us. One of the auxin-binding proteins, a membrane-associated small glycoprotein, seems to participate in, or trigger, the auxin-activated phospholipase A2, since antibodies known to interact with this receptor interfere with the auxin activation. A second non-plant substance, the wasp peptide, mastoparan, was also used to demonstrate the participation of phospholipase A2 in auxin action. This peptide activates plant phospholipase A2 strongly and growth weakly. The weak growth activation may be due to inhibition of the plasma membrane H(+)-ATPase by mastoparan. Other downstream elements of the auxin signal transduction cascade postulated by us are lysophospholipids, which activate a membrane-associated protein kinase that may participate in the regulation of the plasma membrane H(+)-ATPase. One of the natural lysophospholipids in plants, lysophosphatidic acid, activates elongation growth and membrane-associated protein kinase. A continued search for further downstream elements of the auxin signal transduction cascade and improved knowledge of these elements should yield more tools to interfere with and dissect this signal transduction chain.