A gene bank of Sau3A1-generated Streptococcus pneumoniae type 23 DNA fragments was constructed in Escherichia coli K-12 with the low-copy-number cosmid vector pOU61cos. Clone lysates were screened by immunoblotting using a mouse antiserum raised against a crude pneumococcal hyaluronidase preparation. One immunoreactive clone was isolated, and it produced high level of hyaluronidase activity. This clone contained a recombinant cosmid (designated pJCP800) with an approximately 35-kb DNA insert, and the putative hyaluronidase coding sequence was subcloned into pBluescript SK as a 3.8-kb PstI-ClaI fragment (designated pJCP802). The complete nucleotide sequence of this insert was determined. The region included an open reading frame sufficient to encode a polypeptide with an M(r) of 107,751. An active hyaluronidase with an M(r) of approximately 89,000 was purified to homogeneity from E. coli DH5 alpha(pJCP802). N-terminal amino acid sequence analysis of the purified protein suggested that translation initiation was occurring primarily at a TTG codon within the major open reading frame. However, immunoblot analysis using antiserum raised against the purified 89-kDa hyaluronidase indicated that E. coli DH5 alpha(pJCP802) also expressed the 107-kDa form of the enzyme. This antiserum labelled a 107-kDa protein in partially purified hyaluronidase preparations from S. pneumoniae. The hyaluronidase activity in this pneumococcal extract was also neutralized by the antiserum.