Human neutrophil gelatinase was purified to apparent homogeneity. The N-terminal amino-acid sequence of the purified enzyme could be aligned to an internal part of the cDNA-derived amino-acid sequence of 92-kDa type IV collagenase from SV 40-transfected human lung fibroblasts and from a TPA differentiated monocytic cell line, U937. Total amino-acid composition of U937 and neutrophil gelatinases was identical. Gelatinase was susceptible to treatment with o- and n-glycanase, indicating that posttranslational addition of oligosaccharide side chains occurs. An enzyme-linked immunosorbent assay for gelatinase was developed using specific polyclonal rabbit antibodies. The assay was specific, sensitive, accurate, and reproducible. Ninety percent range for plasma gelatinase from normal subjects was 17.3 to 102.9 ng/ml. In patients treated with cytostatic agents for non-Hodgkin's lymphoma, there was a parallel drop in plasma gelatinase and peripheral granulocyte count. This indicates that plasma gelatinase is a marker for circulating neutrophils. Plasma gelatinase does not seem to reflect bone marrow cellularity.