Guinea pig antisera were prepared against purified hexon, penton and fibre antigens of adenovirus type 5. Sera reacting specifically with particular antigens as judged by immunodiffusion, complement fixation and haemagglutination-inhibition tests were assayed for virus neutralizing capacity. The latter was exerted almost exclusively by anti-hexon sera. Type 5 anti-hexon but not anti-fibre sera neutralized type 1 virus provided the serum virus reaction took place at low pH values.