The determination of the absolute number of cells with receptors for sheep erythrocytes and for C3 by rosette techniques are usual clinical tests in the evaluation of the lymphoid populations in different diseases. Antibiotics are commonly added to avoid contamination in the reagents employed in the rosetting studies, as well as in "in vitro" tests of lymphocytes function and in cultures of cell lines. In this study we have established the influence of gentamicin (G), demeclocycline hydrochloride (DH) and ampicillin (A) on the membrane receptors, through their effect on E4 degrees C and EAC formation. We found that in the presence of each of the three antibiotics there was a partial inhibition of both rosettes. The inhibition was found to be dose related, gentamicin concentrations of 85 micrograms/ml and 42,5 micrograms/ml resulting in significant reduction of E4 degrees C and EAC rosettes respectively. Studies with A and DH indicated that EAC rosettes were inhibited by both drugs after incubation times of at least 15 minutes, while a minimum of 30 minutes of incubation was required to attain significant inhibition of E4 degrees C rosettes. These results indicate that the C3 receptors would be more sensitive to the action of these antibiotics than the receptors for sheep erythrocytes.