The biological availability of chloramphenicol from different commercial preparations was investigated. Methods used to evaluate the in vitro release were disintegration, de-aggregation and dissolution tests and particle size measurement. Considerable differences between the rate of release of the antibiotic from the different capsule preparations were detected. These differences can be attributed to the formulation used during the preparation of the different products. The absorption characteristics of chloramphenicol from 3 brands of chloramphenicol capsules and 3 chloramphenicol powder samples from different sources were tested on 6 healthy male subjects in a cross-over trial. Each subject received 500 mg as a single oral dose at intervals of one week. Significant differences between the average amount of chloramphenicol excreted in the urine from the different preparations were noted 1-2 hours after administration of the dose. The experimental results were used to compute the absorption rate of the antibiotic from different preparations. The differences observed in the in vitro release were confirmed by the in vivo results. The differences between the release of the antibiotic from the different capsule preparations could be ascribed to the method of formulation of the antibiotic which is poorly soluble in water.S. Afr. Med. J., 47, 94 (1973)