Streptomyces olivaceoviridis secretes a so far unique protein of 18.7 kDa (CHB1) which lacks catalytic activity. It interacts highly specifically with a-chitin, but not with @-chitin, chitosan, or cellulose. Each of the five codons for tryptophan (Trp) in the chbl gene was replaced by those for leucine (Leu) or tyrosine (Tyr). Eight corresponding mutant proteins and the wild-type protein were purified to homogeneity and their binding capacity to a-chitin was determined. The relative affinities to anti-CHB1 antibodies, the kinetics of binding, the dissociation constants, circular dichroism, and fluorescence emission spectra for three mutant types were compared to the characteristics of CHB1. The presented data lead to the following conclusions.