The proteinase activity of Giardia lamblia trophozoites, Portland 1 strain, was characterized with respect to substrate specificities and inhibitor sensitivities. Proteinase activity with urea-denatured hemoglobin (UDH), α- N-benzoyl- dl-arginine-2-naphthylamide (BANA), and α- N-benzoyl-argininamide (BAA) as substrates exhibited pH optima of 5.8, 3.8, and 5.0, respectively. For BANA, the apparent K m was 0.20 m M and the V max was 2.56 μ M. For BAA, the apparent K m was 4.0 m M and the V max was 8.69 μ M. Dithiothreitol (DTT, 5 m M) enhanced proteinase activity threefold for UDH, fourfold for BAA, and fivefold for BANA. Iodoacetamide, l-tosylamide-2-phenylethyl chloromethyl ketone (TPCK), and N-α- p-tosyl- l-lysine chloromethyl ketone (TLCK), each at 1 m M, inhibited proteinase activity by greater than 90% with BANA and BAA. Iodoacetamide inhibited proteinase activity by 35% with UDH; TPCK and TLCK inhibited activity greater than 70% with UDH. Activity on BAA was inhibited by 91% with Zn 2+ and activity on UDH was inhibited by 30% with Cu 2+. Virtually complete inhibition of proteinase activity on BANA and BAA was obtained with leupeptin and chymostatin at 1 μg/ml. Pepstatin A, chelators, and other heavy metals had no apparent effect on proteinase activity. Two polypeptide bands (ca. 105 and 40 kDa) indicative of proteinase activity were visualized by sodium dodecyl sulfate-gelatin polyacrylamide gel electrophoresis. The 105 kDa band was visible over the pH range of 4 to 7, but with greater intensity from pH 5 to 7. The 40 kDa band, while present at pH 5, was most intense at pH 6 and 7. The proteinase activity of these two bands on gelatin was DTT dependent and was inhibited by 1 m M iodoacetamide, 1 m M TPCK, and 1 m M TLCK, but not by up to 1.2 m M phenylmethylsulphonyl fluoride. These results indicate that the proteinase activities in G. lamblia are primarily of the cysteine type.