This experiment was designed to investigate the in vivo effects of calcium ions on tetracycline absorption when oral doses of tetracycline and calcium-containing medications were administered concurrently. Twenty-one human volunteers were selected and divided into three equal-sized groups: Group A, Group B, and Group C. Each volunteer in Group A received a 250 mg. tetracycline hydrochloride capsule, administered one day with and one day without, a concurrent dose of calcium carbonate. Each volunteer in Group B received a 250 mg. dose of tetracycline syrup, administered one day with and one day without, a concurrent dose of calcium carbonate. Each volunteer in Group C received a 250 mg. tetracycline hydrochloride capsule, administered one day with and one day without, a concurrent dose of calcium gluconate. On each of the two days of medication administration, a series of five blood samples was drawn at selected times from each volunteer. These blood samples were used for subsequent serum tetracycline determinations. Data from individual serum tetracycline determinations was used to compute mean serum tetracycline levels for each of the medication groups. The mean serum tetracycline level data obtained from the medications groups was evaluated statistically by conducting independent sample mean comparisons and paired-sample mean comparisons. The independent-sample mean comparisons revealed that both calcium carbonate and calcium gluconate significantly decreased absorption of tetracycline. In addition, independent-sample mean comparisons revealed that the depressant effect of calcium carbonate on tetracycline absorption was significantly greater than the depressant effect of calcium gluconate on tetracycline absorption. Paired-sample mean comparisons performed at a 95% confidence level revealed no significant difference between effects of calcium carbonate and calcium gluconate on tetracycline absorption. However, when a 90% confidence level was used, calcium carbonate was again found to have a significantly greater depressant effect on tetracycline absorption than calcium gluconate. An explanation for the decreased absorption of tetracycline in the presence of calcium carbonate and calcium gluconate has been offered in terms of calcium ion chelation with tetracycline. An additional explanation, one involving gastric pH influences of calcium carbonate, has been offered as a possible explanation for the significantly greater depressant effect of calcium carbonate on tetracycline absorption. In addition to the statistical evaluation of the effects of calcium containing medications on tetracycline absorption, a clinical evaluation of the experimental data was also conducted. The clinical evaluation involved an examination of the median minimum inhibitory concentrations of tetracycline needed for various microorganisms, and a discussion of the effects which calcium-containing medications may have on obtaining these median minimum inhibitory concentrations.