The objective of this in vitro study was to establish the antimicrobial efficacy and the effect of different activation methods on the smear layer at the coronal level of straight root canals of four different root canal irrigation solutions. The four irrigation solutions were 3.5% sodium hypochlorite liquid (NaOCl), 2% sodium hypochlorite gel, chlorhexidine gluconate liquid and a mixture of 100 mg doxycycline capsules with 2 ml sterile water. ANTIMICROBIAL EFFECTS: The surfaces of four agar plates were inoculated with Enterococcus faecalis and divided into four equal quadrants. Ten microlitres of each test solution was dispensed onto the four filter paper disks on each agar plate. The antibacterial activity of materials was apparent from circular clear inhibition zones forming around the filtration paper. The diameters of these inhibition zones were measured using a micrometer gauge. EFFECT ON SMEAR LAYER: Access cavities were prepared on fifty, extracted, single rooted, human teeth and the root canals prepared with rotary files. The teeth were randomly divided into five groups (n = 10) and each group irrigated with a different irrigation solution. Different activation methods were used in the coronal portion of each root canal. The solutions were activated in the canals using one of the following methods: a 30 gauge needle (Control), a sonic scaler tip, and a rotary brush. After sampling, the roots of the treated teeth were fractured and prepared for Scanning Electron Microscopy (SEM) according to standard methods. The one-way ANOVA test was used to determine whether there were any statistical significant differences between the different groups. The average zones of inhibition for 3.5% NaOCl, 2% NaOCl, 2.5% chlorhexidine and doxycycline were 2.7mm, 2.0 mm, 11.2 mm and 12.4 mm respectively. Sterile water, 3.5% NaOCl and 2% NaOCl had no significant effect on the smear layer. However, when chlorhexidine and doxycycline solutions were activated with a rotary brush, 90 and 80 per cent of the observed surfaces were free of smear layer respectively. Doxycyline and 2.5% chlorhexidine demonstrated the highest antimicrobial activity against Enterococcus faecalis and removed most of the smear layer when the solutions were activated with a rotary brush.