Lymphocytes from Inbred Balb/cyJ mice infected subcutaneously with 10(3) MLD50 of Rickettsia tsutsugamushi Karp strain were unresponsive to rickettsial antigens and phytohemagglutinin in the lymphocyte transformation assay between 11 and 28 days post-infection. There was, however, no correlation between this suppressed responsiveness to rickettsial antigens and the immune capacity to resist a normally lethal ip challenge with the Karp strain. Fourteen days after infection, these mice survived a potentially lethal Karp challenge. Naive recipients of day 14 post-infection mouse splenic lymphocytes also were partially protected against ip challenge. Karp strain-specific anti-rickettsial antibody reached a peak geometric mean titer of 260 during the period of apparent cellular unresponsiveness. However, day 14 post infection mice showed a fourfold decrease of antibody response to a T-dependent antigen, which suggests the presence of T-dependent antibody suppression in these mice.