The effects of multiple infections with Trichinella spiralis on the levels of eosinophil major basic protein (MBP), creatine kinase (CK), and leucocytes were studied in the guinea pig. Plasma MBP levels increased after each of four successive infections with T. spiralis. The times of peak elevations in MBP levels and eosinophilia correlated significantly, and both peaks occurred earlier with increased infection dose. CK levels were elevated most dramatically in the primary infection. A direct correlation between peak CK values and larval dose was observed, and the peak occurred earlier with larger larval doses. A dose-dependent basophilia was observed in the primary infection and, like the eosinophil response, basophilia occurred earlier and was markedly enhanced after the second infection. The elevations in MBP levels and eosinophil counts persisted into the third and fourth infections; the basophil response persisted only through the third infection.