Levamisole, an antihelminthic agent reported to enhance nonspecifically various parameters of the immune response, was examined for its effect on chemotaxis of human neutrophils and on levels of cellular cyclic nucleotides. This agent was found, in most instances, to enhance chemotactic responses of neutrophils to a bacterial chemotactic factor derived from Escherichia coli. At similar concentrations, levamisole produced increases in levels of guanosine 3':5'-cyclic phosphate in neutrophils. In contrast, a decrease in concentrations of adenosine 3':5'-cyclic phosphate was observed when neutrophils were incubated with levamisole. Neutrophil chemotaxis, with and without the addition of levamisole, was assessed in 10 patients with recurrent infections. The illnesses of these patients included Job's syndrome, Wiskott-Aldrich syndrome, eczema with an increased level of IgE and recurrent abscesses, chronic mucocutaneous candidiasis, and diabetes mellitus. Levamisole significantly enhanced chemotaxis of polymorphonuclear leukocytes from these patients. Levamisole appears to have a profound effect on chemotactic responses of neutrophils which probably results from alterations in cellular cyclic nucleotide levels. Levamisole may prove to be useful therapeutically in certain patients with defective neutrophil chemotaxis.