The compounds pyrostatin A and B, isolated from Streptomyces sp. SA-3501 have been reported as N-acetyl-beta-D-glucosaminidase inhibitors with inhibition constants in the micromolar range. Recently, a comparison of NMR spectral data of the pyrostatins has led to a structural revision of the pyrostatins. It was shown that the pyrostatins A and B are identical to the ectoines 5-hydroxectoine and ectoine, respectively. Ectoines are known as compatible osmolytes in many halophilic and stress-tolerant bacteria. We have performed enzymatic experiments demonstrating that neither ectoine nor 5-hydroxyectoine exhibit an inhibitory effect on N-acetyl-beta-D-glucosaminidase. The previously reported inhibition of N-acetyl-beta-D-glucosaminidase by pyrostatins A and B may thus be due to the contamination of the compound preparations with a strong N-acetyl-beta-D-glucosaminidase inhibitor with an inhibition constant (Ki) in the nanomolar range, as has been reported in other Streptomyces species.