Human progastricsin was prepared from extracts of gastric mucosa by chromatography on columns of DEAE-cellulose. The amino acid compositions of progastricsin and gastricsin were determined and calculated on the basis of the molecular weights 38 000 and 32 000 respectively. The activation of progastricsin at pH 2 was investigated and monitored by agarose gel electrophoresis at pH 5.4. Two intermediates were observed. Determination of the amino acid sequence showed that the propart consists of 43 amino acid residues. A pronounced homology with other gastric zymogens was found. With the proenzyme amino acid residue numbering used previously [B. Foltman (1981) Essays in Biochemistry, 17, 52-84] the activation of progastricsin at pH 2 may be summarized as follows. The first cleavage occurs after Phe (p27). At pH 5.4 the peptide remains associated with the protein (intermediate I). Subsequent proteolysis removes the peptides from Leu (p28) to Leu (p45). At pH 5.4 the N-terminal peptide from progastricsin (p2-p27) remains associated with gastricsin (intermediate II) until the propart peptide is hydrolysed to smaller fragments.