Chemical estimation of seven different marketed brands of tetracycline/Hcl capsules for tetracycline content showed six brands were not meeting the Indian pharmacopia 1985 prescribed standards. However, their dissolution rate and disintegration time were in accordance with United States pharmacopia 1985 specifications. One standard and one substandard brand of tetracycline were selected and their bioavailability, after single dose and plasma steady-state levels after multiple dosing in well nourished and undernourished subjects were determined. The bioavailability of substandard product as determined from 48 h urinary tetracycline excretion was significantly lower as compared to standard product both in well-nourished and undernourished subjects. The plasma steady-state concentrations with the substandard product were below the generally recommended minimum inhibitory concentrations, more so in undernourished subjects. These results indicate that the use of sub-standard tetracycline products in undernourished subjects may lead to therapeutic failures and/or result in the development of resistant microorganisms.