A clearance method for ceftizoxime (CZX), a new cephalosporin antibiotic, was designed and utilized to assess the mechanism of renal excretion and evaluate kidney distribution under steady-state serum level. Renal clearance ratio of CZX to inulin, when corrected for protein binding, was 1.6 at serum level of 13 micrograms/ml and was decreased to around unity with a serum level of 700 micrograms/ml or higher or by infusing p-aminohippuric acid or probenecid. The clearance ratio appeared to be independent of urinary pH or changes in urinary flow. The kidney/serum concentration ratio for CZX decreased from 2.5 to 1.5 in proportion to the extent of reduction of the clearance ratio. p-Aminohippuric acid and probenecid further reduced the kidney/serum ratio to about 1.0 with concomitant inhibition of tubular transport of CZX. Cortex/serum and medulla/serum ratios were also reduced similarly to that of the whole kidney. It is concluded that the tubular transport of CZX is similar to that for p-aminohippuric acid transport system and that passive reabsorption from the tubular lumen may not necessarily be present.