The Cathepsin B-like proteinase, a secretory form of lysosomal cathepsin B, is present in some cancerous exudates (i.e. pleural and ascitic fluids). It has been suggested that this enzyme may be involved in the invasive process, one of the most important aspects of cancer pathology. In the same fluids, two kinds of cysteine-proteinase inhibitors (C.P.I.'s) are found- HMr-CPI (90,000 daltons) and LMr-CPI's (11,000-13,000 daltons). The Cathepsin B-like enzyme is more strongly inhibited by the LMr-CPI's than by the HMr-CPI, the Ki values are 4.0 X 10(-9) M and 1.2 X 10(-7) M respectively. Theses results underline the similarity between this enzyme and the lysosomal Cathepsin B. On the other hand, the inhibitors could play a protective role against tumor invasion.