Serologic evidence is presented for the existence of rhinovirus antigens with C and D type reactivity, and for their loss after acid treatment. No neutralizing antibody could be detected in animal antisera prepared with pH 3.0-treated rhinovirus antigens. In contrast, antisera prepared with untreated (native) rhinovirus antigens, contained high levels of neutralizing antibody. In immunodiffusion tests, no precipitin lines were detectable when native antigens were diffused against antisera prepared with pH 3.0-treated antigens. Acid treated antigens also were non-reactive by immunodiffusion when reacted against antisera prepared with treated or untreated antigens. The diffusion of native antigens against hyperimmune animal sera prepared with untreated antigens gave one precipitin line. With normal human sera, two precipitin lines were observed with untreated antigens. Rhinoviruses and enteroviruses were found to share a common group reactive antigen.