The antibacterial activity of innovative, commercial lining cements was investigated. A liner which contains calcium hydroxide and is polymerized by visible light (Prisma VLC Dycal) and a glass-ionomer lining cement (GC lining cement) were compared with two more established lining cements (Advanced Formula II Dycal and Life). Antibacterial activity and hemolysis-like agar change at 24, 48, and 72 hours were measured on blood agar plates inoculated with Streptococcus mutans KPSK 2 (serotype c), Lactobacillus casei ssp rhamnosus ATCC 11981, and chewing-stimulated saliva. Prisma VLC Dycal did not affect bacteria or agar. The glass-ionomer lining cement, with an acidic pH at setting, had the most pronounced effect on all test organisms and on the agar. Even after 48 hours' setting, it inhibited growth of S. mutans. The control lining cement (AFII Dycal) showed antibacterial activity toward both specific micro-organisms as well as some activity against the salivary organisms. The material Life showed only partial inhibition of microbial growth. For all lining cements, the hemolytic-like agar change correlated with antibacterial effects. The surface pH of the freshly-set cements containing calcium hydroxide was alkaline. It would seem that a simple correlation between high surface pH and antibacterial activity among these cements does not exist. Also, further biological characterization of new lining cements is required to direct their appropriate clinical use.