Growth of Borrelia burgdorferi in a modified Kelly-medium is described. Borrelia strains were isolated from patients (n = 11) and ticks I. ricinus (n = 19). The modified medium which contained Co-trimoxazole is a very effective medium for isolating and culturing of Borrelia sp. The susceptibility of 7 strains of B. burgdorferi to antibiotics was studied by the macrodilution and microdilution test. After preliminary testing for optimal conditions, we determined MICs in modified Kelly medium. The MIC concentration of each antibiotic was determined as the lowest concentration which completely inhibited growth of the tested organism. The B. burgdorferi was most susceptible to Erythromycin with MIC of less than or equal to 0.15 microgram/ml. Of the Penicillins tested, Ampicillin and Mezlocillin were more active than Penicillin G. The use of Tetracycline-HCl is recommended because of its low MIC in vitro its extra- and intracellular efficiency.