Oral doses of 125, 250, and 500 mg of a new cephalosporin, CGP 9000 {7β-[D-2 -amino-2-(1,4-cyclohexadienyl)-acetamido]-3-methoxy-ceph-3-em-4-carboxylic acid}, given to healthy volunteers, were evaluated pharmacokinetically. The mean peak concentrations with the respective doses were 4.5 ± 1.4, 7.7 ± 2.7, and 12.3 ± 4.2 μg/ml. The maximum serum concentration occurred after 0.5–1.5 h. The amounts recovered unchanged in urine during 12 h were 96.4 ± 5.1, 92.4 ± 11.7 and 96.7 ± 7.6% of the respective doses. There was straight linear kinetics with a doubling of area under the serum curve for each dose, rising from 6.1 with the lowest, through 12.1 with the middle dose to 23.6 μg·h/ml with the highest dose. There was an apparent lag of 0.3–0.4 h before absorption. Absorption was relatively rapid, with mean absorption rate constants of 4.68–6.26 h-1. Serum half-life varied between 0.9 and 1.0 h with the different doses. The mean renal clearance values with the three doses ranged between 322