An in vitro system for chemotherapeutic research using adult male Onchocerca gutturosa has been developed as a model for O. volvulus. Using a culture system consisting of medium MEM+10% heat inactivated foetal calf serum (IFCS)+LLCMK2 (monkey kidney) feeder cells in an atmosphere of 5% CO2 in air, we examined the effects of a range of antiparasitic drugs on worm motility. Ivermectin, levamisole, furapyrimidone, Mel W, chloroquine, metrifonate, flubendazole, amoscanate and the Ciba-Geigy compounds CGP 6140, CGP 20′376 and CGI 17658 either immobilized or significantly reduced motility levels at a concentration of 5x10−5M or less within a 7-day period. Worms were affected at very low concentrations by ivermectin (effective conc. to reduce motility levels to 50% of controls, 3.14x10−8M), levamisole (7.95x10−8M), CGP 6140 (8.87x10−9M) and CGP 20′376 (2.78x10−8M). Difficulties were experienced in accurately repeating the immotile endpoint for levamisole due to an inconsistent partial recovery of motility. Over a 7-day period diethylcarbamazine had little effect on motility levels, while suramin caused a slight increase in activity compared to controls at some timepoints. Subsequent experiments demonstrated some differences in drug efficacy depending on the presence or absence of serum and feeder cells in the culture system probably because of drug avidly binding to serum proteins. However, serum and cells were found to be essential ingredients of the culture system to maintain worms in good condition, indicating that new drugs should be evaluated both in the presence and absence of serum and cells. Comparisons were made between the responses of O. gutturosa and Brugia pahangi to certain drugs and these species were found to significantly differ in their sensitivities to ivermectin and a novel compound (Wellcome), indicating that Onchocerca parasites should be used wherever possible for compound identification and development intended for the treatment of onchocerciasis. The in vitro system described here, using male O. gutturosa, provides a basis for further research and a practical alternative to O. volvulus.