The advantages of a membrane filter system for blood culturing have been realized for many years. Lysing of the blood prior to filtration is a convenient way to proceed, but previously described lysing procedures result in loss of certain organisms, particularly gram-negative bacilli. Four concentrations of Triton X-100 and sodium carbonate were studied in vitro, and their lysing and antibacterial properties were observed. A solution of 0.08% Na2CO3 and 0.005% Triton X-100 was found to have the least antibacterial effect and gave consistently good lysis and filtration times (under 3 min). An 8.3-ml amount of blood added to 190 ml of this concentration of lysing solution, filtered through three 47-mm membrane filters (0.45-mum pore size), led to recovery of 85% or more of various aerobic and facultative organisms in studies of artificially seeded blood.