The equivalency of drug products is being questioned with increasing frequency. Oxytetracycline hydrochloride 250-mg capsules produced by the 11 manufacturers supplying the US market were compared in a series of two-way crossover studies with 19 to 22 subjects each. The product (original manufacturer's) associated with the most clinical experience was used as reference product. Serum concentrations of seven comparison products were more variable and markedly lower (P < 0.005) than the reference product two, three, and six hours after ingestion.