Gelatinase B is a matrix metalloproteinase involved in tissue remodelling. When mouse cells are triggered in vitro with interleukin-1, bacterial endotoxin, virus-mimicking double-stranded RNA or cytokine inducers, they produce gelatinase B. To test the effects of gelatinase B in vivo, the enzyme was expressed in Chinese hamster ovary (CHO) cells. Hybrid genomic DNA-cDNA constructs under the control of two constitutive viral promoters were generated by PCR-mediated exon amplification. In vitro transcription and translation of the mRNA in reticulocyte lysate yielded the correct 79-kDa protein, and expression in CHO cells resulted in an intact glycosylated 110-kDa gelatinase B which was enzymically active. However, the production yields of recombinant enzyme from 50 tested clones were low and cell-culture supernatants contained significant amounts of copurifiable endogenous CHO gelatinase B. Therefore, the enzyme was expressed in the yeast Pichia pastoris. Recombinant proenzyme was secreted and recovered from the yeast culture medium at 10 mg/l. Amino-terminal sequence analysis indicated that affinity purification of the recombinant protein on gelatin-Sepharose yielded the expected N-glycosylated proenzyme form (110 kDa) in addition to an amino-terminally truncated unglycosylated variant (69 kDa). Both forms had gelatinolytic activity on zymography. The recombinant mouse gelatinase B was used to determine its pharmacokinetics and its haematological effects in vivo. After intravenous injection in rabbits, gelatinase B disappeared from the circulation within 6 h. In addition to a transient leukopenia, we observed a rapid increase in leukocytosis, which indicates that gelatinase B might be a factor involved in the desorption of adherent leukocytes from the vascular bed and in the release of leukocytes from the bone marrow. Gelatinase B secretion and activation might well be one of the crucial molecular mechanisms explaining leukocytosis which is associated with infections and almost all types of inflammation.