The gene encoding suilysin was cloned from Streptococcus suis serotype 2 strain P1/7. Analysis of the nucleotide and translated amino acid sequence confirmed suilysin to be a member of the thiol activated cytolysin group (TACY). The pneumolysin from Streptococcus pneumoniae is the most closely related orthologous gene known. Suilysin was overexpressed in E. coli in an active haemolytic form. A strong correlation between the presence of the sly gene and haemolytic activity in the supernatant of S. suis field strains was found. Of 158 strains tested, 63% contained the gene. Within the (most prevalent) serotype 2, the sly gene was demonstrated in 95% of the strains isolated in Eurasia, but only in 7% of the strains from North America.