A panel of seven different groups of monoclonal antibodies (Mabs) specific for the 15 kD nucleocapsid protein of porcine reproductive and respiratory syndrome virus (PRRSV) was generated from mice that were immunized with antigens derived from PRRSV isolate ATCC VR-2402 (ISU-P). These groups, designated A through B, were determined by their reactivity in the IFA to 67 different field isolates representing 13 states,Canada,the European Lelystad virus and the current North American vaccine strain, ATCC VR-2332. The field isolates were collected during 1989 through 1995. Group A Mab’s reacted with all field isolates and the Lelystad virus and as such should be considered for use as universal diagnostic Mab’s for PRRSV. The field isolates and the Lelystad virus were able to be categorized into seven distinct groups (I-VII) based on their reactivity to the panel of Mab’s. Group I represented 56 isolates. The six remaining groups were represented by one to four field isolates each. These observations demonstrate that there is a wider degree of diversity among North American PRRSV isolates then was previously believed.