Although type 2 antigens, such as PVP, generally do not activate specific TH, previous studies have established that low doses of PVP (0.0025 microgram) can activate TH in vivo which provide help in primed B cells for PVP-specific IgG responses. Doses of PVP that are optimally immunogenic for IgM antibody production (0.25 to 25 micrograms) preferentially activate PVP-specific TS, which suppress IgG antibody production. In the studies reported here, TH and TS that regulate PVP-specific IgG antibody responses were activated in vitro by culturing normal spleen cells for 4 days with PVP. Induction of the TH and TS is dependent upon the amount of PVP in culture: 10(-4) micrograms PVP activates TH, whereas 10(-2) micrograms PVP preferentially activates TS. TH induced in vitro express Thy-1, L3T4, and I-A determinants and help provided by these TH is similar in magnitude to that provided by TH from mice primed with 0.0025 microgram PVP in vivo. TH can also be activated in vitro if donor mice are treated with Cy before culture of their spleen cells with 10(-2) micrograms PVP. Cy pretreatment prevents TS activation, and TH are then induced in these cultures. The presence of TS does not prevent activation of TH by 10(-2) micrograms PVP, because removal of TS by treatment of T cells with anti-Lyt-2 + complement at the end of culture uncovers TH activity. This TH activity is comparable with that of TH obtained after culture with 10(-4) micrograms PVP. The ability to activate PVP-specific TH and TS in vitro should allow determination of the mechanisms involved in activation of T cells by type 2 antigens and the mechanisms by which TS and TH interact with one another.