Microfilariae were studied in skin and lymph node biopsies from Liberian patients with generalised onchocerciasis 12-78 hours after administration of a single dose of 150 micrograms/kg body weight using histology, transmission electron microscopy and immunocytological staining with antibodies against an immunodominant antigen of Onchocerca volvulus. Most microfilariae in the skin appeared morphologically intact and beginning signs of degeneration were seen only on the ultrastructural level. The densities of microfilariae in the lymph nodes were about thousandfold higher in ivermectin treated patients. More than 90% of the microfilariae in the lymph nodes showed distinct signs of degeneration. Early changes were seen in the muscle cells. The disintegrating microfilariae in the lymph nodes were always encircled by eosinophils or macrophages or both cells. Immunohistological staining with antifilarial antibodies increased the detection of small and disintegrating pieces of microfilariae considerably.