Infectious viral particles production in PRRSV-infected SJPL cells following five consecutive passages. PRRSV IAF-Klop strain was passaged serially in MARC-145 and SJPL cells as described in the materials and methods section. The amount of the infectious viral particles recovered after each passage was determined in MARC-145 cells. The virus titers were expressed as TCID50 per 106 cells. The initial viral inoculum (inocul) used to infect both cell lines was 103.3 TCID50/106cells.