The role of neutrophils and eosinophils in the acquired resistance of mice to infection with the helminth Nematospiroides dubius has been investigated in vivo by testing infected mice for their resistance to a challenge dose of larvae following treatment with monoclonal rat antibodies (MAbs) specific for mouse granulocytes. Treatment of normal and infected mice with NIMP-R10 MAb reduced the number of neutrophils in the peripheral blood and peritoneal cavity during the ensuing 2-5 days without affecting macrophage or eosinophil levels. The effect of this MAb on immunity depended on the immune status of the mice tested. The partial resistance of mice receiving a primary 'immunising' infection, followed by passive transfer of immune serum and challenge at 4 days, was completely suppressed by NIMP-R10 MAb. The acquired resistance of mice challenged 10 days after being given a single 'immunising' infection was halved by NIMP-R10 treatment, while that of 'twice-immunised' mice (infected on days 0 and 14, challenged on day 24) was unaffected. Treatment of twice-infected mice with the eosinophil-specific MAb NIMP-R6 reduced the number of eosinophils in the blood and peritoneal cavity by 40-60%, but caused no significant loss of immunity. The data indicate that after infection of mice with N. dubius, neutrophils play a predominant role in early resistance to reinfection, but they become progressively less essential as activated macrophages and (following a second infection) eosinophils become prevalent. The lack of effect of NIMP-R6 MAb treatment on the immunity of twice-infected mice may have been due to an insufficient reduction in eosinophil numbers; however, it seems likely that the neutrophils and macrophages present in these mice would have provided adequate protection even in the absence of eosinophils.