Ocular onchocerciasis is the second most common infectious cause of blindness in the world. Th-2 helper T (Th-2) cells are thought to play a critical role in immediate hypersensitivity against allergens and extra-cellular parasitic infections. Th-2 cells secrete a high amount of IL-4, regulate IgE production, and recruit eosinophils and mast cells. Conjunctival biopsies from ten African patients with ocular onchocerciasis were evaluated for the presence of II-2, IL-4, mast cells and major basic protein (MBP), a marker for eosinophils. IL-4 mRNA was detected in seven of ten conjunctival specimens using in situ hybridization, yet IL-4 was detected in only one specimen using immunohistochemical staining. In contrast, IL-2 mRNA was detected in three of ten conjunctival specimens and IL-2 was detected in two specimens. There were greater numbers of mast cells and the presence of MBP in specimens with IL-4 mRNA. Furthermore, the three biopsies containing both IL-2 and IL-4 mRNA had greater numbers of CD4+cell infiltration and the patient with IL-4 protein in his conjunctiva also had the highest IgE in his aqueous humor. These findings suggest that Th-2 cells and their lymphokines are important for the localized host responsiveness to ocular onchocerciasis.