Plant cells respond to elicitors by inducing a variety of defense responses. Some of these reactions are dependent on the activity of protein kinases. Recently, mitogen-activated protein kinases (MAPKs) have been identified to be activated by fungal and bacterial elicitors as well as by pathogen infection. In gel kinase assays of alfalfa cells treated with yeast cell wall-derived elicitor (YE) revealed that 44- and 46-kDa MAPKs are rapidly and transiently activated. Immunokinase assays with specific MAPK antibodies revealed that YE mainly activated the 46-kDa SIMK and the 44-kDa MMK3 and to a lesser extent the 44-kDa MMK2 and SAMK. When cells were treated with chemically defined elicitors potentially contained in the YE (chitin and N-acetylglucosamine oligomers, beta-glucan, and ergosterol), the four MAPKs were found to be activated to different levels and with different kinetics. Whereas SIMK and SAMK have been found to be activated by a number of diverse stimuli, MMK3 is activated during mitosis and was therefore assumed to participate in cell division (). No physiological process could be associated with MMK2 activity so far. This is the first report that MMK2 and MMK3 can be activated by external stimuli. Overall, our findings indicate that plant cells can sense different cues of a given microorganism through the activation of multiple MAPKs.