-We studied the morphogenesis of three pseudorabies virus mutants lacking parts of the gene homologous to the UL21 gene of the herpes simplex virus type 1. The mutants were examined in an SK-6 cell-line, in an SK-6 cell-line expressing the UL21 gene product, in porcine lung alveolar macrophages (PLAM) and in porcine nasal mucosa explants. Although on SK-6 cells and PLAM, the virus-assembly and egress of mutant virus M155, lacking almost the entire UL21 gene, was similar to that of the rescued PRV mutant, M155 producing virions containing little or no DNA (Atype particles). Virus mutants M133 and M134 (lacking 23 and 232 amino acids respectively) produced more C-type particles. In SK-6 cells stably expressing the UL21-encoded protein, all mutants produced C-type particles. All mutants produced C-type particles in nasal mucosa explants, indicating that the UL 21-gene product is not essential for virus production in porcine tissue. These results support and extend previous work that indicated a role for the UL21 encoded protein in the packaging of newly replicated viral DNA. electron microscopy / morphology / pseudorabies virus / UL21 gene Résumé -La délétion du gène UL21 du virus de la maladie d'Aujeszky résulte dans la formation de capsides dépourvues d'ADN : étude en microscopie électronique. Nous avons étudié la morphogenèse de trois mutants du virus de la maladie d'Aujeszky (VMA) comportant des délétions dans l'homologue du gène UL21 du virus de l'herpes simplex de type 1. Les mutants ont été examinés dans une lignée cellulaire SK-6, dans une lignée SK-6 exprimant le produit du gène UL21, dans des Vet. Res. 32 (2001) 47-54 47