Lanthanum ions (La3+) drastically changed the pH dependence of the photocurrent responses from bacteriorhodopsin (bR). The pH value of the photocurrent polarity reversal was shifted to lower pH and the ratio of the light-off to the light-on photocurrent peak intensities, I-off/I-on, was significantly decreased, compared with the photoresponse from the wild-type bR. These results are explained by invoking the effect of La3+ on the bR proton pump activity.