The capacity of measles virus and haemagglutinin (HA) and fusion (F) glycoproteins, presented either as soluble antigens, associated with liposomes or as Iscoms, to induce an in vitro primary and anamnestic cellular response was studied in syngeneic W/Fu rats using the MTT colorimetric assay. A primary cellular response was observed when the virus, HA + F liposomes or HA + F Iscoms were used as immunogens, but not when soluble HA + F glycoproteins were used. The irradiation of the naïve spleen cells at 500 rads allowed the generation of a primary response with the soluble antigens. All these primary responses were low, of similar intensity and dose-dependent. The responses were stronger after the stimulation of spleen cells from seropositive immune rats with virus, HA + F liposomes or HA + F Iscoms, whereas they were moderate after stimulation with soluble HA + F. In addition, far less antigenic material was required and the use of a vehicle for HA + F (liposomes, Iscoms) dramatically lowered the threshold of the sensitivity to the antigens. The immunization of rats with soluble HA + F glycoproteins resulted in the anergy of their spleen cells even to virus, HA + F liposomes or HA + F Iscoms. Again, irradiation of these cells could restore their ability to elicit a primary response to any type of HA + F immunogens. Using the lysosomotropic Leu-0-Met agent, all the cellular responses were found to be accessory cells dependent, the responses being restored after supplementation with 10% peritoneal exudate cells from naïve rats. These treatments did not break the immunosuppression induced by soluble HA + F glycoproteins. The uptake of the various immunogens by the murine macrophage cell line J774-1 was also studied using radioactively labelled virus and HA + F glycoproteins. The uptake of soluble HA + F was limited to 10-15%, whereas that of the other immunogens was almost complete. The data reported indicate that the modification of the supramolecular architecture of HA + F glycoproteins by their presentation in liposomes or Iscoms could modulate their immunogenicity, both qualitatively and quantitatively. It could prevent the generation of a radiosensitive suppressive mechanism, increase the sensitivity to the antigen and the activation level of the responding cell population. Quantitative modifications are accessory cell dependent and initiated within the first hour of the stimulation.