Changes of protein and RNA synthesis due to the effect of penicillin, streptomycin, chloramphenicol and oxytetracycline were studied in suspensions of human tonsillar cells invitro. Inhibition of macromolecular synthesis occurred at concentrations of about 1 mg/ml of the first three antibiotics mentioned and at about 5 µg/ml of oxytetracycline. The inhibitory effect of penicillin and oxytetracycline develops moreslowly than that of streptomycin and chloramphenicol and is reversible