Nine pigeon paramyxovirus type 1 isolates from the United States and Canada were characterized and three of the isolates were pathotyped before and after passage in chickens and serial passage in chicken embryos. One isolate previously passaged in Madin Darby bovine kidney cells was also pathotyped after chicken and embryo passage. Hemagglutination (HA) titers of all isolates were low when tested by microtiter procedures and all were negative by rapid-plate HA. The HA titers were increased by a factor of 8 to 32 by Tween-ether treatment, and treated antigen had the same reactivity as untreated antigen in hemagglutination-inhibition (HI) tests. All isolates had a slow elution rate and an HA thermostability equal to or greater than 60 minutes. Mean death times in embryos were 99 hours or greater, except for one isolate with a mean death time of 81 hours, and intracerebral pathogenicity indices of all isolates were greater than 1. Antigenic differences among the pigeon isolates were identified by three different binding patterns in HI tests against a battery of five Newcastle disease virus (NDV) monoclonal antibodies. Pathogenicity enhancement by bird, embryo, or cell passage was limited to an intravenous pathogenicity index increase for one of three viruses passaged in embryonated eggs. Cloacal samples collected during chicken passage contained higher virus titers than did oral samples. The pigeon isolates reported here, like those of earlier reports, have properties that prevent characterization within a single NDV pathotype. Finally, there was no evidence that any of these isolates was highly virulent for chickens.