A method of predicting the virulence of a new or uncharacterized PRRS virus strain is provided wherein the strain is injected into swine and allowed to replicate for a period of from about 3-15 days. During this period, the rate of virus growth and/or the magnitude of viremia is determined, and this data is compared with a corresponding growth rate and/or viremia magnitude of a PRRS virus strain of known virulence, as a measure of the virulence of the new or uncharacterized strain.