Due to the threat of bioterrorism, large-scale clinical trials of a new cell culture smallpox vaccine were conducted. Biologically false positive (BFR) serological reactions to viruses (hepatitis B and C, HIV) and syphilis were evaluated. BPR rapid reagin tests (RPR) to syphilis occurred in 19% and false positive tests for antibody to hepatitis B in 3.3% of 90 healthy adults undergoing primary vaccination. Most subjects (94%) were RPR-positive on Day 15 after vaccination and all seroreverted within 2 months thereafter. One subject with myocarditis was RPR-negative. One RPR-positive and 1 RPR-negative subject had elevated CK-MB enzymes without other evidence for myocarditis.