SummaryImmunodepression phenomena in infections with protozoan and metazoan parasites have raised a great deal of interest because of the apparent lack of immunological control in a primary infection. Therefore, this phenomenon has been proposed as a working hypothesis to explain the successful association. However, the role that immunodepression can play in parasitic infections as a mechanism to assure an equilibrium between the host and its parasite has never been evaluated. The immunodepression to SRBC in Trichinella‐infected mice was removed by treating mice with low doses of cyclophosphamide (CY). The elimination of immunodepression resulted in a significantly lower number of encysted muscle larvae. In the absence of immunodepression, a higher number of circulating eosinophils was counted. The results of these experiments are discussed firstly with regard to the mechanisms which could explain immunodepression in trichinellosis and secondly, the role that immunodepression plays in the relationship between the host and the parasite during a primary infectio