Adult Nematospiroides dubius excretory-secretory products (ES) were collected from worms cultured in vitro, separated by sodium dodecyl sulphate--polyacrylamide gel electrophoresis (SDS-PAGE) into four fractions (FI-IV), electroeluted and assessed for their ability to inhibit the proliferation of mouse lymphocytes stimulated by mitogens in vitro. The proliferation of mitogen- and ES-stimulated mouse spleen lymphocytes from normal and infected mice was inhibited by low mol. wt ES F-IV (less than 26,000).