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The effects of tannic acid (TA) on acid phosphatase (APase) (E.C.3.1.3-2) from root extracts of fertilized or unfertilized Raphanus raphanistrum plants were examined. TA inhibited APase activity non-competitively; Vmax was reduced, but the apparent K^ was unaffected in the presence of added TA. Endogenous tannin content ranged between 0.26 and 0.56 (Imoleg fresh weight * of root tissue. Enzyme activity data was extrapolated to provide estimates of activity in the absence of any TA; uncorrected APase activity of R. raphanistrum was underestimated by an average of 25%. Addition of polyvinylpyrrolidone (PVP) did not affect measured enzyme rates in a consistent manner.