Troubleshooting addendum added June, 2009:

Eliminating False Positive Bands from the Activity Stain:

When the activity stain is applied to agarose or polyacrylamide gels, artifactual colorless bands may appear under certain conditions during Step 5 of the procedure.  In this Step, the color has developed to the desired extent and activity bands are visible, so the gel is soaked in water to wash out any unreacted NBT, stopping the further development of color.  The gel background should remain purple, due to the precipitation of the insoluble formazan on the gel matrix.  If, however, there are bands of protein at very high concentration (such as albumin in plasma samples, or hemoglobin in red cell lysates) the formazan may bind to these proteins rather than to the insoluble gel matrix.  During this washing step, the soluble proteins may be eluted from the gel, carrying with them the bound formazan and thereby creating a clear zone that may be mistake for SOD activity.  One identified and understood, these artifactual clear bands may simply be ignored.  Alternatively, the gel may be imaged without this washing step once the desired color development has been achieved.  Keep in mind that if the unreacted reagents are not removed by washing, the gel will eventually turn very dark, so the imaging must be done promptly once optimal color development is achieved.