The sporocidal properties of peracetic acid, hydrogen peroxide, chlorine, and formaldehyde were comparedin vitrousing a dilution–neutralization micromethod. A combination of peracetic acid and hydrogen peroxide was also tested to assess their interactions. The activities of these agents, which are widely used as disinfectants, were evaluated againstBacillusspore isolates found on stored membranes and collection cultures. Peracetic acid and chlorine exhibited an excellent antimicrobial activity, with a destruction of 105spores/mL after 5 min of contact. Generally the effects of the biocides tested were time dependent. The sporocidal activities of hydrogen peroxide and formaldehyde were the lowest. The combination of peracetic acid and hydrogen peroxide, tested by a checkerboard micromethod, was found to be synergistic. The minimal sporocidal concentration (MSC) was established in terms of time for each biocide. The lowest MSC values for peracetic acid, hydrogen peroxide, chlorine, and formaldehyde were 168–336 ppm (1–2 h of contact), 5625–11250 ppm (5–7 h), 168–336 ppm (2–3 h), and 1875–3750 ppm (5–30 min), respectively. The MSC of a biocide combination of peracetic acid and hydrogen peroxide showed that synergy was maintained with increasing contact time and that the MSC could be reduced by two to eight times when compared with those of the biocides alone. Optimal concentrations and contact times of those chemicals that were promisingin vitrowere then tested for their ability to disinfect ultrafiltration membranes. The sporocidal activities of peroxide compounds and chlorine were confirmed and the synergism between peracetic acid and hydrogen peroxide was also maintained. The biocide combination of peracetic acid and hydrogen peroxide gave successful results; 21 ppm of peracetic acid + 2813 ppm of hydrogen peroxide allowed a total disinfection of hollow fibers in 2–3 h of contact.Key words: peracetic acid, hydrogen peroxide, chlorine, combination, ultrafiltration membranes.