We have recently demonstrated the occurrence of functional circulating soluble cell-free Fc gamma 2b/gamma 1 receptor in normal mouse serum by means of the 2.4G2 rat monoclonal antibody. With the solid phase radioimmunoassay described previously, we report here a dramatic increase in the level of circulating soluble cell-free Fc gamma 2b/gamma 1R in Schistosoma mansoni-infected mice individually tested before and on days 22, 41, 62, and 82 after infection. This increase was statistically highly significant when the five measurements from each mouse were compared, and also when the entire group of infected mice was compared with the stable level of Cf-Fc gamma 2b/gamma 1R measured in strain-, sex-, and age-matched control mice individually tested on the same days. This increase was commensurate with a rise in the amount of IgG detected in the sera of the S. mansoni-infected mice. Thus, infection seems to be one of the factors that modulate the expression of such soluble Cf-Fc gamma 2b/gamma 1R in mouse serum. Furthermore, the experimental device reported here for the production of high levels of Cf-Fc gamma 2b/gamma 1R by infection (in this case by parasitic infection) could serve as a model for obtaining large quantities of serum Cf-Fc gamma 2b/gamma 1R for further purification. Lastly, we point out that, by establishing a functional relationship with circulating IgG, such soluble Cf-Fc gamma 2b/gamma 1R may modulate some of the functions in which the Fc portion of immunoglobulins is involved.