In this study, we describe the time course and T-cell dependence of the serum antibody response to the periodontopathogen Porphyromonas (Bacteroides) gingivalis in an experimental rat mode. Normal Fischer rats were challenged by a local injection of P. gingivalis (2 x 108 bacteria) into gingival tissue or by the administration of a similar number of bacteria by the intravenous (i.v.) route on days 0, 2 and 4. Serum antibody activity was detected within 1 week and peaked at 8 weeks after gingival challenge. A similar but lower response was seen for rats challenged by the i.v. route. The response in both groups of rats was mainly of the immunoglobulin G (IgG) isotype; some IgM but no IgA antibody activity was detected. Analysis of the IgG subclass revealed mainly IgG2c in animals challenged locally in the gingiva with P. gingivalis, whereas IgG2b predominated in rats challenged by the i.v. route. The importance of T cells in the response was established by demonstrating the absence of serum IgG antibodies in nude rats after a local challenge of gingival tissue with P. gingivalis. Nude rats given purified splenic T cells from normal rats immunized systematically with P. gingivalis prior to local gingival challenge showed a rapid appearance of serum antibody activity that peaked between 4 and 6 weeks. This initial peak occurred 2 to 4 weeks earlier than that seen in normal animals. Fluorescence-activated cell sorted analysis of splenic lymphoid cells from these nude rats revealed a helper T-cell population. The levels of serum IgG antibodies in nude rats given nonimmune T cells rose slowly, and the antibodies were mainly of the IgG2a and IgG2b subclasses. Nude rats given immune T cells showed a rapid increase primarily in IgG2b antibody levels following a local gingival challenge. These findings suggest that the immune helper T-cells contributed to the rapid development of the response of P. gingivalis. Furthermore, it is likely that the IgG subclass response to P. gingivalis in these nude rats was related to the splenic origin of the T cells used for adoptive transfer.