Erythromycin and oxytetracycline uptake by the giant freshwater prawn, Macrobrachium rosenbergii , was evaluated by the two-step osmotic infiltration technique. Test prawns were immersed in 7.5% NaCl conditioning (hyperosmotic) solution for 30 min. then transferred to a freshwater (hypo-osmotic) solution containing 10 mu g/ml of antibiotic. Antibiotic uptake was determined in test and control prawns at 5, 10, 20 and 40 min. Maximum concentration of erythromycin was observed in test prawns 10 min. after exposure, whereas the same concentration was not achieved in control prawns until 40 min. Oxytetracycline uptake was similar in treated and control prawns in that maximum tissue levels were observed after 40 min. in both cases. The uptake of erythromycin was significantly greater in prawns treated for 10 min. than in prawns treated for 5, 20 and 40 min. Osmotic infiltration did not significantly enhance oxytetracycline uptake at all time intervals tested.