The serological property and replication in swine alveolar macrophages (SAM) and MARC-145 cell cultures of 35 porcine reproductive and respiratory syndrome (PRRS) viruses isolated in SAM were investigated. All the isolates were reacted almost equally with antisera against three Japanese isolates including EDRD-1 strain (American type) in immunoperoxidase monolayer assay (IPMA), but weakly or did not react with antiserum against Lelystad virus (European type) indicating that the Japanese isolates are more closely related to an American type virus. Twenty two of 35 isolates replicated with CPE both in SAM and MARC-145 cells, whereas remaining 13 isolates did not show CPE in MARC-145 cells. The antigenicity of the isolates did not relate to the virus origin and the replication in the cell cultures.