When Dictyostelium discoideum amoebae were harvested from nutrient medium and suspended in a starvation buffer to initiate development, approximately 30% of the total cellular beta-N-acetylglucosaminidase activity was secreted into the extracellular fluid within 4 h. During this same period, only 10% of the total cellular acid phosphatase and acid protease activities were secreted. When the cells were pretreated overnight with 5 mg sodium hadacidin ml-1 and then suspended in starvation buffer, 60% of the glucosaminidase and 30% of the acid phosphatase activities were secreted, while the level of acid protease secretin remained at 10%. The secretory behaviour of hadacidin-treated cells was, however, identical to that of untreated cells when 0.1 M-sucrose was added to the starvation buffer to enhance lysosomal enzyme secretion. Treatment with hadacidin also affected the intracellular content of these enzyme activities. After 16 h exposure to 5 mg hadacidin ml-1, the cellular levels of glucosaminidase and acid protease activity were decreased by 50% and 30% respectively, while acid phosphatase activity remained unchanged. All of the changes observed upon hadacidin treatment were time dependent and were not evident if the cells were exposed to the drug for only 4 h. These results suggest that hadacidin treatment affects the lysosomal system of D. discoideum.