Five samples of ampicillin capsules with a label claim of 250 mg were purchased from different dispensing points in a small town in Nigeria. The pharmaceutical quality of the products and a sample from a batch produced by a local manufacturer was evaluated and five of the capsule samples were employed in an in vivo bioavailability study. Three of the five capsule samples from dispensing points were found to be of lower quality than the officially prescribed standards of pharmaceutical quality. The quality lapses observed were sufficient to bring about determinable differences in biological availability. The results demonstrate that ampicillin capsules of sub-standard chemical quality are being dispensed within the study sources from authorised and unauthorised sources and that this may have biological, clinical and epidemiological consequences.