The absorptions of 6 erythromycin preparations were compared in a cross-over study in healthy humans. In a single-dose study, 500 mg of each preparation was, after an overnight fast, given to 10 volunteers. The two enterosoluble preparations of erythromycin base studied were absorbed slowly, and the peak serum concentration (1.5-2 mg/l) was achieved only at 4 h. The absorption of the stearates was quick, but especially one of them was poorly absorbed, the serum concentration being always below 1 mg/l. Both of the two estolates gave highest apparent concentrations, and the maximum serum level (2-2.5 mg/l) was achieved at 2 h, but the concentration of active erythromycin remains unknown. In the second part of the study, two erythromycin stearates and one base preparation were given at 6-h interval in a cross-over fashion, each for 4 days. On the 4th day, blood samples were analyzed. The erythromycin base gave higher serum concentrations than did the two stearates, which were equivalent. It seems doubtful that the erythromycin stearate at the dose of 250 mg every 6th hour would give satisfactory serum levels of erythromycin which would be effective against most bacteria during the whole treatment.