The hepatitis B virion is a spherical double-shelled particle carrying three surface proteins (large [L], middle [M], and small [S]) in its envelope. All three proteins are translated from a single open reading frame by means of three different in-frame start codons from unspliced mRNAs. This organization defines three protein domains (pre-S1, pre-S2, and S). All three domains together form the L protein, whereas the M protein consists of domains pre-S2 plus S. The L and S proteins are both necessary for virion production, whereas the M protein is dispensable, suggesting an important function of the pre-S1 domain in virion morphogenesis. To investigate this point, we created a series of N-terminal-truncated L mutants and tested their ability to substitute for the wild-type L protein in virion formation. We found that the constructs fell into two classes, (i) N-terminal deletion mutants lacking up to 102 of the 119 amino acids of the pre-S1 domain still allowed virion maturation, showing that the N-terminal 5/6 of the pre-S1 sequence is dispensable for this process. (ii) Mutants lacking 110 or more N-terminal amino acids were unable to substitute for the L protein in virion assembly, although they were stably expressed and secreted as components of subviral 20-nm hepatitis B surface antigen particles. This suggests that a short C-terminal region of pre-S1 is important for virion formation. Like the wild-type L protein, the mutants of the first class were not glycosylated in their pre-S2 domains; however, this site was used for glycosylation in mutants of the second class, similar to that in the M protein. These findings can be related to a model for the function of the L protein in virion maturation.