A procedure dating back to the early penicillin era is adapted in order to determine the activity of the new cephalosporins ceftizoxime and ceftriaxone against Treponema pallidum in vitro. While the well-known activity of penicillin G is confirmed for the virulent Nichols strain (0.002 micrograms/ml lead to 50% immobilisation) the new cephalosporins turn out to be almost as efficacious. The concentration of ceftizoxime leading to 50% inhibition amounts to 0.004 micrograms/ml. The corresponding figure for ceftriaxone is 0.01 micrograms/ml. The potential importance of these findings for the treatment of syphilis in man are discussed.