The major physiological role of human secretory leukocyte protease inhibitor (SLPI), a low molecular weight inhibitor present in mucus, is the rapid formation of a tight-binding inhibitory complex with neutrophil elastase. It is also the most effective known inhibitor of human mast cell chymase. The inhibitory efficacy of recombinant SLPI towards three other mast cell chymases was therefore investigated. Rat mast cell proteinases-1 and -2 (rMCP-1 and -2, respectively) and sheep mast cell proteinase-1 (sMCP-1), a chymase with additional tryptase-like properties, were treated with the inhibitor. SLPI inhibited rMCP-1 very efficiently in the absence of heparin, with a low dissociation constant, K_i=3×10⁻¹⁰ M and high second order association constant, k_ass=8.0×10⁶ M⁻¹ s⁻¹, and inhibition was enhanced when heparin was present. rMCP-2 was not inhibited by SLPI in the presence or absence of heparin, and did not degrade SLPI on prolonged incubation. SLPI inhibited sMCP-1 very poorly in the absence of heparin (K_i=9×10⁻⁶ M). However, in the presence of heparin, the K_i for inhibition of sMCP-1 by SLPI was reduced to the nanomolar range. sMCP-1 was observed to cleave SLPI with chymase-like specificity at Leu⁷²–Met⁷³ on prolonged incubation in the absence of heparin, but increasing concentrations of heparin reduced the extent of cleavage.