The chromatographic behaviour of some cephalosporins as a function of pH and ionic strength of the mobile phase was studied on 10-microns LiChrosorb RP-18. Acidic cephalosporins were retained longest in their neutral form with an acidic eluent. Amphoteric cephalosporins were retained longest in their protonated form with an acidic eluent of low ionic strength. Cefotiam was retained longer with an alkaline mobile phase. LiChrosorb RP-18, Nucleosil C18 and muBondapak C18 gave rise to different selectivities when an acidic eluent, methanol-water (25:75) containing 0.2% of 1.8 M H2SO4 was used. This may be related to interactions with residual silanol groups. The studied cephalosporins (with the exception of cefotiam and cefsulodin) were separated from compounds present in biological fluids on 5-microns LiChrosorb RP-18 using the mobile phase 0.2% of 1.8 M H2SO4 in a mixture of methanol and water with various methanol contents. The determination of cefotiam in biological fluids was performed with an alkaline mobile phase. The preparation of the sample was simple and rapid: precipitation of plasma proteins or dilution of urine. The method was applied to the determination of ceftizoxime in human plasma and urine. Concentrations down to 0.2 micrograms/ml of plasma and 25 micrograms/ml of urine could be determined with good reproducibility and accuracy.