The antigenic relationships among Hart Park (HP) virus strain AR 70, Flanders virus strain 61-7484, and other related strains were investigated. Strains AR 70 and 61-7484 were shown to be antigenically related but different from each other in neutralization, complement fixation, and double-diffusion tests. Two strains isolated in Texas and one strain each, isolated in Illinois and Florida, appeared to be closely related if not identical to Flanders virus (61-7484) and antigenically related but different from HP virus AR 70. Ten additional strains isolated in 10 different states were compared to the two prototype strains. Seven of the 10 could not be distinguished from Flanders virus, and all 10 strains were different from HP virus. The results indicate that a virus-type consisting of viruses antigenically related if not identical to Flanders virus is widely distributed within the United States, whereas HP virus is antigenically related but different from this type.