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Iron starved dicotyledonous plants undergo several modification aimed at increasing iron assimilation, including enhanced plasma-membrane linked redox activities in roots (1). Monocots are known to regulate their iron requirement mainly by release of phytosiderophores. However a plasma-membrane associated Fe3+-ehelate reduetase activity exists in maize and is modulated by iron starvation. This increase is partly a consequence of the more abundant plasma membrane material found as part of an iron starvation syndrome affecting also root morphology (fresh mass +30 %). Molecular mass determinations of solubilized plasma-membrane or microsomes by means of FPLC size-exclusion chromatography identify a 210 kDa protein in both control and stressed material solubilized with 2 % (m/v) lysopbophatidylcholine. This protein appears to be very sensitive to freezing-thawing, high salt and dilution. After exposure to such conditions the activity is partially recovered (25 %) associated to 43 and 26 kDa forms tentatively identified as subunits of the 210 kDa (see also refs. 2,3). Other detergents directly give rise to such low molecular mass forms. These are also found in the supernatant fraction.