Despite its wide use, the plate clearing assay based on CMC–agar is notorious for its low specificity, producing halos around other polymer-degrading enzymes like amylase and agarase (Figure 1A), independent of the presence or absence of a cellulase substrate (e.g., CMC) (Figure 1B). The artifacts occur likewise with other gelling agents like agarose and Gelrite (Figure 1C) and with other staining methods like Congo Red (data not shown). Among these problems, particularly the substrate-independent formation of halos seriously impacts one of the most important criteria of enzyme assays, their specificity and precludes any chances to quantify cellulase activities. A quarter of a century ago, Zitomer and Eveleigh [10] already recommended to exercise caution when interpreting results using iodine stained CMC–agar plates. They found that small amounts of contaminating starch in commercial agars made the staining unreliable. Their concerns have been substantiated and extended now by the controls shown here (Figure 1). The use of CMC–agar (or –agarose or –Gelrite) in connection with Gram’s iodine staining as a stand-alone method can thus not be regarded as reliable.