Purified soluble hexon antigens of type 5, 7, and 8 adenoviruses, belonging to three different subgroups have been studied. The electrophoretic mobility of the three hexons was found to be different in immunoelectrophoresis, and the calculation of absolute electrophoretic mobility gave different values too. Significant dissimilarities were observed in the elution patterns of the hexons in anionexchange chromatography. However, no essential differences between the molecular weights of the hexons were demonstrable, they appeared to be with all three types about 300,000. Determination of the smallest hexon quantities demonstrable by immunodiffusion, immuno-osmophoresis and complement fixation (CF) reaction, showed the CF reaction to be the most sensitive. 0.02 μg of type 5 and type 8 hexons and 0.08 μg of type 7 hexon were detectable using homologous anti-hexon sera. The anti-hexon sera neutralized the cytopathic effect of the homologous adenovirus types only. In the immunodiffusion experiments any two antigens of the three proved to be identical, examined with the third (heterologous) serum. Using homologous serum they revealed only a partial identity. Absorption experiments were successful only when homologous antigen was used, even excess amount of heterologous hexons failed to remove the antibodies from the anti-hexon sera. These results demonstrate the presence of closely connected group-, subgroup or type-specific components in the hexons of adenoviruses, and definite differences in the physico-chemical properties of hexons belonging to different serotypes.