Incubation with diethylcarbamazine citrate caused significant augmentation of human neutrophil and eosinophil adherence to tissue culture plastic. This effect was dose dependent and cell dependent, with eosinophils showing greater sensitivity and a greater adhesive response to the drug than did neutrophils. Eosinophils preincubated with diethylcarbamazine citrate demonstrated decreased adhesive responses to other adherence-augmenting stimuli. Use of Fc-treated plastic augmented diethylcarbamazine citrate-stimulated neutrophil (but not eosinophil) adherence. Direct stimulation of host effector cell adherence may explain, in part, the therapeutic action of diethylcarbamazine citrate in vivo.