Three strains of cloned porcine adenoviruses (25RCl, 6618Cl and A47C1) were shown to be serologically distinct in cross neutralization tests with specific antisera prepared in rabbits. They were not neutralized by reference antisera against the known human adenoviruses. Neutralizing antibodies to each of the three serotypes were demonstrated in samples of pig serum. Guinea pig, human 0, rat, monkey and mouse red cells were haemagglutinated by strain 25RCl, but fowl, ox, pig, rabbit and sheep cells were not haemagglutinated. Strains 6618Cl and A47Cl did not haemagglutinate these cells. Specific rabbit antiserum and normal pig sera contained HI antibodies to strain 25RCl adenovirus. Each of the three strains was resistant to ether and pH 4.0 buffer and growth in PK cells was inhibited by FUDR. In infected PK cells stained with haemotoxylin and eosin, typical basophilic central nuclear masses were produced by each strain of porcine adenovirus. In PK cells infected with strain 6618Cl multiple eosinophilic intranuclear inclusions in some cells were a prominent feature of the CPE. The provisional designation of strains 25RCl, 6618Cl and A47Cl as porcine adenovirus types 1, 2, and 3, respectively, is proposed.