Interaction of eglin c with three neutral proteinases (1, 2A and 2B) from horse leucocytes was investigated using synthetic and protein substrates. With N-tert-butyloxycarbonyl-L-alanine-p-nitrophenyl ester as substrate inhibition of proteinase 1 and 2A was practically complete at equimolar inhibitor concentrations (Ki below 1 nMol/l). The complex with proteinase 2B showed a dissociation constant of approximately 25 nMol/l. The latter proteinase was only partly inhibited also in the presence of azocasein, whereas almost linear inhibition was observed for all 3 proteinases with fibrinogen as substrate. The inhibition rate constants (kon) for horse leucocyte proteinases with eglin were in the range of 8 to 13 X 10(5) M-1 S-1.