The phytotoxin, helminthosporal (H-al), can damage the permeability of plant cell membranes and it is suggested that this may be an important factor in the susceptibility or predisposition of cereal tissues to attack by Bipolaris sorokiniana, the fungus which produces the toxin. Helminthosporal was tested for its effect on the apparent free space (AFS) of barley roots and the efflux of betacyanin from red beet root tissue. Cell membrane disruption was indicated by an increase in the AFS of H-al-treated barley roots. The effect of H-al on the AFS of barley roots did not appear to be correlated with respiratory inhibition by the toxin since cyanide, azide, and 2,4-dinitrophenol failed to increase the AFS. Oxygen uptake by red beet root tissue was strongly inhibited by H-al at concentrations of 1.0 mM and 2.0 mM, and was accompanied by an immediate and large efflux of betacyanin. No correlation was found, however, between the extent of respiratory inhibition and the amount of pigment released. Helminthosporal reacts directly with both the plasmalemma and the tonoplast membranes of the beet root cell, exclusive of effects on respiration, since the loss of betacyanin was equally rapid under aerobic or anaerobic conditions. The integrity of the beet cell tonoplast membrane appeared to be unrelated to respiration and energy supply. Calcium ions, which act to stabilize membranes, partially alleviated the loss of pigment from red beet tissue exposed to n-propanol and H-al. Permeability change in the plant cell membrane may be as important in the mode of action of H-al as are the effects of the toxin on respiration and oxidative phosphorylation.