Highly purified soybean (Glycine max L. Merr.) plasma membranes exhibit a lipoxygenase activity with a pH optimum in the acidic (5.5-6.0) range and with a Km value of 200 microM for both linolenic and linoleic acids. This activity is inhibited by nordihydroguaiaretic acid (NDGA), salicylhydroxamic acid (SHAM) and propyl gallate, stimulated by CaCl2 up to 0.25 mM, H2O2 (5 to 10 nM range) and by some nucleotide triphosphates (125 to 1000 nM range) in the following order ATP > GTP = UTP > CTP. The enzyme is not released by treatment of the membranes with 0.05% Brij 58 and its activity is approx. 65% inhibited by the impermeant p-chloromercuryphenyl-sulfonate only in 0.01% Triton X-100-treated membrane vesicles. These results indicate that soybean cells have an acid lipoxygenase, associated to the plasmalemma, with the catalytic site on the cytoplasmic surface. It may be distinguished from the soluble counterpart, because the latter is not stimulated by nucleotide triphosphates. The plasma membrane vesicles also show a lipoxygenase, active in the alkaline (9.0-9.5) range, inhibited by NDGA, SHAM and propyl gallate, stimulated by H2O2, but with a lower Km value (60 microM) and less sensitive to calcium stimulation than the acidic one. The possible involvement of acid lipoxygenase in senescence and in the response of plant cells to wounding and pathogen infection is discussed.