Infective larvae, adults and newborn larvae of Trichinella spiralis were surface labelled with radioactive iodine, and the surface material was solubilized in the mild detergent sodium deoxycholate. The radio-isotope labelled products were stage-specific glycoproteins that were few in number (2–4 components) and antigenic in infected mice and rats. Antibodies synthesized in infected animals against these biochemically defined surface antigens may or may not interact with the surface of the living worm. The latter type of antibody is unlikely to be involved in the initial phase of parasite rejection and is therefore another example of a non-protective host antibody response. The stimulus for its synthesis must be the observed release of surface antigen. A monoclonal antibody to a surface glycoprotein of newborn larvae protected against infection, and also promoted eosinophil killing in vitro. This observation emphasizes the importance of surface antigens in protection against infection, suggests a role for granulocytes in vivo, and provides encouragement for the possible use of nematode surface antigens in protection. An example of regional specialization of the nematode cuticle was given by a monoclonal antibody reactive with only the surface of the male intromittent organ and not the female or remainder of the male. The same stages were labelled in vitro with radioactive methionine, and the secreted proteins were also found to be stage-specific. Some, but not all, were antigenic in infected mice. The total concanavalin A-binding somatic glycoproteins of each stage exhibited considerable individuality, and hence stage specificity, when resolved by two-dimensional gel electrophoresis. These results extend our knowledge of stage-specific components of T. spiralis, and allow a rational approach towards protection and the construction of diagnostic procedures.