Major capsid proteins (MCPs) of various papillomaviruses have recently been expressed in heterologous cells as soluble and functional polypeptides. The host cells for producing these proteins have so far been of eukaryotic origin; however, E. coli has potential utility a host, with advantages over eukaryotic cells such as relatively simple culture requirements and greater ease of mutation of expressed sequences. We studied the expression by E. coli of the MCP of human papillomavirus type 16 (HPV16) using the gene derived from the 'prototype' HPV16 genome. Using expression vector pTrc99A, the protein was produced in full-length unfused form at levels of 3-4% of cell protein. Soluble polypeptide was detected, albeit at low levels. The level of solubility was not increased by growing cells at low temperature and slowing the rate of protein synthesis. The soluble protein was degraded at its carboxy terminus by an outer membrane protease of E. coli, OmpT, giving rise to two slightly shortened protein species of 52K and 56K in addition to the full-length 57K polypeptide. Since the MCP of prototype HPV16 is known to be prone to excessive aggregation compared with other papillomaviral MCPs, the recovery of soluble polypeptide indicates that E. coli is worth consideration as an alternative host to eukaryotes for producing these proteins.