In this experiment, the influence of sodium bicarbonate and magnesium oxide on the absorption of minocycline (MC), cephalexin (CEX), chloramphenicol (CP) and ampicillin (AB-PC) from the gastrointestinal tract after oral administration to humans and rabbits was investigated. Serum levels of the antibiotics were determined by thin layer horizontal diffusion method with Sarcina lutea PCI-1001 in the test on CP and Bacillus subtilis PCI-219 in others. The absorption of MC was apparently inhibited by the antacids administered simultaneously, but when MC was given 6 hours after the administration of the antacids, the decrease in the absorption was not seen. Even if CEX or CP was given simultaneously with the antacids, the absorption of the antibiotics was not inhibited at all, but the decrease in the absorption of AB-PC was apparently observed after the simultaneous administration of magnesium oxide.