The morphogenesis of a Dutch PRRS field strain virus (Lelystad virus) was studied and compared to that of a U.S. field strain VR2332 and its attenuated vaccine strain JJ1882. Porcine lung alveolar macrophages (PLAM) and CL2621 cells were infected with high doses of virus (MOI = 10). At 4, 6, 9, 12, 18, 24, and 48 h post infection (hpi) cells were fixed for electronmicroscopy or for detection of viral antigens by immunoperoxidase staining. From 6 hpi on, viral antigens were detected in the cytoplasm and from 9 hpi on completely assembled virus particles could be detected in infected cells. The three strains were similar in assembly of new virus particles, envelopment at the smooth endoplasmic reticulum, and egress from infected cells. However, distinct differences were seen in replication time of the three strains in various cell types. The Lelystad virus replicated very fast and efficiently in PLAM while VR2332 and JJ1882 replicated preferably in CL2621 cells. JJ1882 replicated faster in CL2621 cells than VR2332 did, probably because of increased adaptation to the cell-line. Although the U.S. and European strains differ at the level of the genome, morphogenesis is not visibly altered. There is however a distinct difference in preferred cell type between the European strain and the two U.S. strains.