The response to dinitrofluorobenzene (DNFB), bovine serum albumin (BSA), and sheep red blood cells (SRBC) was studied in CBA mice infected with Trypanosoma musculi at days ranging from 3 to 63 days of infection. DNFB contact sensitivity was normal. In 3-, 10-, and 15-day infected mice, anti-BSA hemagglutinating antibody titers were lower than in noninfected controls when determining after 21 days of BSA challenge, and were normal in sera collected after 10 and 14 days. In 14 day infected mice, anti-SRBC indirect plaque-forming cells (PFC) (per million of nucleated spleen cells and per total spleen) were strongly diminished, but anti-SRBC direct PFC were diminished only per million of nucleated spleen cells. In T. musculi recovered mice, the response to DNFB, BSA, and SRBC was normal.