Problem statement: Cyclodextrin complexation has previously been shown to improve the solubility and dissolution properties of trimethopr im; however, no report provides an account of the e ffect cyclodextrin complexation has on the antibacterial activity of this agent. Approach: β-cyclodextrin and 2- hydroxypropyl β-cyclodextrin inclusion complexes of trimethoprim were prepared and confirmed by differential scanning calorimetry and proton nuclea r magnetic resonance. The in-vitro antibacterial activity, in terms of minimum inhibitory concentrations, of c yclodextrin-drug complexes were compared to uncomplexed free trimethoprim by a broth-microdilution method against several sensitive and resistant Gram-positive and Gram-negative bacteria. The effect of complexation on the apparent permeability coefficients was also determined using a Caco-2 per meability assay to account for potential alteration s in bioavailability that could influence in-vivo antibacterial activity. Results: Inclusion complexation of trimethoprim with both unsubstituted and hydroxylat ed versions of β-cyclodextrin produced a reduction in the MIC 80 required to inhibit the growth of S. aureus ATCC 29213, S. pneumoniae ATCC 4961, S. epidermidis ATCC 14990 and E. coli ATCC 25922 (p>0.05). The effect was limited to bacteria normally susceptible to trimethoprim. Neither complex negati vely affected the antibacterial activity of trimeth oprim. Hydroxypropyl-β-cyclodextrin and β-cyclodextrin inclusion complexes significantly (p< 0.01) increased the apparent intestinal permeability of trimethoprim by 39.8 and 56.1%, respectively. Considering the effe ct cyclodextrin inclusion complexation has on the anti bacterial activity of trimethoprim, the improved intestinal permeability of these complexes has the potential to improve the in-vivo antibacterial activity of the agent by enhancing the steady-state concentrati on of the drug when dosed orally. Conclusion: These results would suggest that physical complexation wi th either of these cyclodextrins would provide a feasible strategy to improve the pharmaceutical and pharmacokinetic properties of trimethoprim.