279A supernatant from eggs of the ruminant nematode Trichostrongylus colubriformis contained an enzyme that was similar to leucine aminopeptidase (LAP), based on hydrolysis of the substrate L-leucine ,8-naphthylamide to ,8-naphthylamine. A Michaelis-Menten constant (Km) of 0.155 mM was obtained. Rate of hydrolysis of 16 substrates revealed that L-phenylalanine and L-tyrosine ,8-naphthylamides were hydrolyzed most readily while seven additional substrates were hydrolyzed at lesser rates. The optimum pH for enzymatic activity was 6.75-7.5. Enzymatic activity was lost by heating the egg supernatant to 60°C for 5 min or freezing at 0°C for 28 days. Addition of millimolar concentrations of the chlorides of zinc, manganese and magnesium to the egg supernatant had no stimulatory effect on enzyme activity while 10 and 100 mM concentrations significantly reduced activity. Ethylenediamine tetraacetic acid at 10-4 M had no effect on enzymatic activity. Activity was inhibited by 10-4 M 1,10-phenanthroline, but the inhibition was reversed by zinc chloride at 10-3 M. Di-isopropylphosphofluoridate at 10-3 M reduced enzymatic activity moderately. Enzyme activity in egg supernatant increased 2.2-fold from 21 days to 60-90 days of a primary infection in the host while a 3.3-fold increase was found in primary versus secondary infections.