Nestling budgerigars and galahs and one-day-old SPF chickens were inoculated with an homogenate prepared from the feathers of a variety of birds with psittacine beak and feather disease (PBFD), and known to contain virus-like particles 20 nm in diameter. Uninoculated birds were included as in-contact controls and groups of birds were also inoculated with homogenate treated with beta-propriolactone to inactivate any virus present. Typical PBFD developed in many of the inoculated birds and in some in-contact controls but in none of the birds inoculated with inactivated homogenate nor in SPF chickens. It is concluded from these findings that PBFD is an infectious disease of viral aetiology and is identical to the disease in budgerigars commonly referred to as 'French Moult'.