A structurally highly ordered arrangement of the polyprotein precursor Pr55gag is a necessary prerequisite for assembly, budding and maturation of the human immunodeficiency virus type 1 (HIV-1). In particular, distinct regions of the matrix protein (pi7) and the capsid protein (p24) contained within Pr55gag are functionally active during these processes. In order to determine such regions we exchanged amino acid triplets within pi7 (amino acids 46-61) and p24 (amino acids 341-352) for alanine residues and deleted the whole regions. Synthetic peptides derived from these regions had been shown previously to inhibit the production of infectious virus. The effect of the mutations on the release of viral particles was investigated by using recombinant baculoviruses for the expression of mutated Pr55gag as virus-like particles and by use of the respective HI proviruses for monitoring the production of infectious particles.