Newcastle disease virus (NDV) possesses two envelope spike glycoproteins: the haemagglutinin-neuraminidase (HN) protein and the fusion (F) protein. The HN protein, which is responsible for virus attachment to sialic acid-containing receptors, varies in length due to differences in the sizes of the ORFs. An HN protein precursor of 616 aa has been found in avirulent but not in virulent NDV strains, whereas an HN protein of 571 aa can be detected in highly virulent strains only. An HN protein of 577 aa is present in virulent and avirulent strains. The F protein, which mediates virus-cell fusion, requires proteolytic activation at an internal cleavage site, whose amino acid composition determines cleavability by various proteases. Here, the functional significance of the length of the HN protein in combination with F protein cleavage sites typical for virulent (velogenic and mesogenic) or avirulent (lentogenic) strains was investigated. To this end, site-directed mutagenesis was used to construct recombinant NDV on the basis of an infectious clone of the lentogenic vaccine virus Clone-30. Only recombinant NDV expressing an F protein with a multibasic cleavage site typical of virulent strains was able to spread efficiently in cell culture, irrespective of the size of the HN protein. Moreover, as determined by the intracerebral pathogenicity index (ICPI) in 1-day-old, specific-pathogen-free chickens, pathogenicity was influenced by the cleavability of the F protein and not by the length of the HN protein. The maximum ICPI value obtained for these recombinants was 1.3, as compared to a possible maximum of 2. This demonstrates that the modifications introduced did not result in the conversion of the lentogenic Clone-30 to a velogenic strain with an ICPI value of >1.5 and suggests the involvement of additional virulence determinants that contribute to the pathogenicity of NDV.