Combinations of antimicrobial agents are often used in medical practice. The combination therapy aims a broader spectrum of chemotherapy and/or synergistic effects. The effect of combination of chemotherapeutics on bacteria may be described as synergistic, indifferent or antagonistic. Therefore, it is essential to test the effectivity of antimicrobial combinations in vitro. The checker-board-technique is the most frequently used test to assess antibacterial combinations. For this standard test macroscopically read an incubation time of 18-24 hours is necessary. To reduce this incubation time to 4 hours a rapid nephelometric method was developed. The results of this method showed a good correlation with the standard test.