The glucan elicitor from cell walls o f the fungal pathogen, Phytophthora megasperma f. sp. glycinea, caused a decrease in activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase in wounded soybean cotyledons, whereas wounding alone led to an increase in the activity of this enzyme. A decrease o f HMG -CoA reductase activity after elicitor treatment was also found in soybean hypocotyls and soybean cell cultures. In contrast to the activity o f HMG -CoA reductase, the activity of dimethylallylpyrophosphate: 3,6a,9-trihydroxypterocarpan dimethylallyltransferase increases after elicitor challenge of soybean cell cultures and after inoculation of soybean hypocotyls (cv Amsoy 71) with mycelium of either race 1 (incompatible) or race 3 (compatible) of P. megasperma.