Of 7 hybridomas which secrete immunoglobulins binding to crude extracts of Schistosoma japonicum adults and/or eggs in solid-phase radioimmunoassays (RIAs), 3 gave positive precipitation reactions in the circumoval precipitin test (COPT). Using a competitive RIA with an egg antigen extract and a labelled COPT-positive hybridoma ascites fluid, PwF.41-1-3, only one anti-egg antibody specificity appeared to be represented in the series of 2 antibodies (as ascites fluids). Using sera as inhibitors in the competitive RIA, inhibitory activity (presumably antibodies to the target antigenic determinant of PwF.41-1-3) was readily detected in sera from egg-immunized mice and was of relatively high titre in a strain of mouse (C57BL/6) which can be readily sensitized for large granuloma formation around entrapped eggs in the lungs. Negligible inhibitory activity was found in the sera from S. japonicum-infected patients, even with sera from patients with prominent hepatosplenomegaly. The availability of COPT-positive hybridoma antibodies should facilitate isolation of at least one S. japonicum egg antigen involved in COP reactions and perhaps induction of immunopathological immune responses at least in mice. [AS]