The kinetics of formaldehyde washout in new and reused dialyzers with 2% (w/v) and 4% formaldehyde solution was studied. Using a standard method of rinsing, the concentration of formaldehyde decreases exponentially, but the rate of decrease and the steady-state level depends on the type of dialyzer. The residual quantity of formaldehyde using a 4% solution is more than twice that seen with a 2% solution in similar dialyzers. Bacteriological tests on water-adapted, formaldehyde-resistant organisms indicate that a significant proportion of these organisms can survive a 4-hr incubation with 4% formaldehyde at 20 degrees C. However, increasing the temperature of incubation to 40 degrees C or the addition of ethanol up to 8% (v/v) improved the bacteriological efficacy of formaldehyde. Under these conditions, a 1% formaldehyde solution allows eradication of all organisms tested. Similar results were observed in multiple-use dialyzers inoculated with the same organisms. There was no effect of incubation at 40 degrees C on the in vitro clearance determinations of new and reused dialyzers.