Pathogenicity index tests on 40 paramyxovirus type 1 viruses isolated from field cases of disease in racing pigeons produced a close grouping of the values obtained in intracerebral pathogenicity index ( ICPI ) tests in day-old chicks (mean 1.48, range 1.28 to 1.76) but a much greater variation in intravenous pathogenicity indices ( IVPI ) in six-week-old chickens (mean 0.38, range 0 to 1.81), although 25 out of 40 isolates had an IVPI in the range 0 to 0.3. Isolates with low IVPIs in chickens gave much higher values in intravenously infected adult pigeons. Virulence for chickens was increased by passage through chickens and, regardless of initial IVPI , indices of about 2 were recorded for isolates passaged three to four times. Passage through chickens also increased the virulence for pigeons in the one isolate tested. Pigeons inoculated with isolate 561/83 by intranasal route or by placing birds in contact with infected birds were shown to excrete virus and respond immunologically but no clinical signs were recorded. Isolate 561/83 was shown to readily infect chickens placed in contact with infected pigeons but no disease signs were seen. The clinical signs associated with field disease in racing pigeons could be reproduced in experimental pigeons by intravenous inoculation of virus. Similar clinical signs were seen in chickens infected by the intravenous route.