1. The capacity of recombinant human secretory leukocyte proteinase inhibitor (SLPI) to inhibit human leukocyte elastase (HLE) and cathepsin G (Cat G) was investigated and compared with a recombinant truncated form (carboxyl-terminal domain, Asn55-Ala107) called 1/2 SLPI. 2. Both compounds were efficient when tested against enzymatic activities of purified HLE and Cat G indicating that the HLE- and Cat G-inhibitory sites were preserved in the truncated form. SLPI and 1/2 SLPI also affected platelet activation induced by 0.2 microM Cat G (IC50 = 112 +/- 13 nM for SLPI and 280 +/- 12 nM for 1/2 SLPI). 3. The effects of SLPI and 1/2 SLPI were then tested against polymorphonuclear neutrophil (PMN)-mediated platelet activation, a cell-to-cell interaction mediated by HLE and Cat G released from PMN. In this experimental system, addition of SLPI or 1/2 SLPI before N-formyl-Met-Leu-Phe (fMLP) led to the inhibition of the resulting platelet activation. As was the case for Cat G enzymatic activity and Cat G-induced platelet activation, SLPI was more efficient than 1/2 SLPI (IC50 = 676 +/- 69 nM vs 1121 +/- 150 nM). 4. The ratio of the IC50 against PMN-mediated platelet activation compared to purified Cat G-mediated platelet activation was 6.03 for SLPI and 4.32 for 1/2 SLPI. This difference may be due to the smaller size of the truncated form which could allow this molecule to diffuse more easily between PMN and platelets. 5. In conclusion, 1/2 SLPI could be a promising candidate in the treatment of pathological states linked to inflammation in which participation of HLE and Cat G has been evoked.