The growth characteristics, including ceiling temperatures for growth, of three strains of monkeypox virus were studied on chick chorioallantoic membrane, growth and plaque formation were studied in RK13 cells, and growth was studied in rabbit dermis. The three strains could not be distinguished by these tests but could be differentiated from variola, vaccinia, and cowpox viruses. Haemagglutination-inhibition tests with homologous and heterologous antisera also showed that the monkeypox strains were indistinguishable, although they could be differentiated from vaccinia and cowpox, but not from variola, viruses. Similar results were obtained in neutralization tests. It is suggested that the monkeypox strains constitute a homogeneous poxvirus entity. The strains can be differentiated from other poxviruses by their cultural characteristics but serologically they are more closely related to variola than to vaccinia or cowpox viruses.