Eosinophil differentiation activity has been identified using a simple assay to detect eosinophil differentiation in vitro. Two factors were involved in this activity: eosinophil differentiation factor (EDF, Mr 32-64K) and IL-3 (Mr 19-44K). In this paper it is shown that eosinophil differentiation activity (EDA) can be detected in the serum of mice undergoing eosinophilia induced by Mesocestoides corti. This serum activity is shown to follow the ability of spleen cells to produce EDF after stimulation with parasite antigen or pokeweed mitogen. The activity in mitogen stimulated spleen supernatant (MSSS) and serum has a mean Mr of 38K (range 26K-58K). IL-3 is detectable in MSSS but not in serum. The appearance of the EDA is accompanied by an increase in eosinophil precursors in the bone marrow. These reach a peak at about eight to 16 days. A significant blood eosinophilia was detected by 16 days, reaching a peak at 24 days, although blood levels are a poor indicator of the number of eosinophils reaching the tissues. Eosinophils were present in large numbers in the spleen by 14 days and in the peritoneal exudate by 21 days. At peak levels, 5 X 10(7) eosinophils could be recovered from the peritoneal exudate.