Cysteine is a relatively infrequent constituent of proteins, which in its thiol or half-cystine form contributes in a special manner to their three-dimensional structure. We show that in small cystine-containing peptides, the Cys content is always higher than the average in proteins in general. This observation makes it possible to search for new peptides by monitoring only their Cys content. We have developed a chemical assay for the detection of cyst(e)ine-rich peptides in tissue extracts. Using this assay we have isolated from porcine intestine a novel cysteine-rich peptide, which we denote ZF-1. ZF-1 is homologous to a single zinc-finger motif and has an acetylated N-terminus. This is the first demonstration of the existence of a processed single zinc-finger-like structure. The structural homology of ZF-1 to the zinc-finger motif, present in several metal-binding and DNA-binding proteins, suggests an important role of this peptide in metal transport and/or modulation of gene expression.