Includes bibliographies. / The effect of a 0,2 μm air venting in-line filter on the incidence of post-infusion phlebitis was studied in a prospective, controlled, observer-blind investigation of 132 infusions. The filters were changed daily and the infusions were allowed to continue until no longer required, or until there was a reason for discontinuation. Microbiological evaluation of filters, skin at the site of cannulation immediately prior to removal of cannulae, and cannula tips was performed. Particle size analyses of the commonly used intravenous infusion fluids and medicines was also carried out. The incidence of phlebitis was significantly reduced by the inclusion of the filter in-line. The efficacy of the filter in reducing phlebitis was most pronounced when intravenous antimicrobial agents, especially the cephalosporins, were administered. Micro-organisms were isolated from 13% (28/209) of filters which originated from 31% (16/32) of infusions with filters in-line. A relationship could not be established between the number of filters used per infusion or the number of intravenous additives to the infusion system. Phlebitis was not associated with microbial contamination of cannula tips. Medicines for intravenous administration, especially the powders to be reconstituted prior to administration, were heavily contaminated with particulate matter prior to filtration.