Tiamulin hydrogen fumarate has been shown to be highly active in vitro against Mycoplasma hyosynoviae, an organism that causes arthritis in pigs. A gilt-multiplier herd with a history of this condition was selected to evaluate the efficacy of tiamulin in vivo for the treatment of this disease. The presence of M hyosynoviae was confirmed by its isolation from two typically affected cases. A field trial was carried out on clinically affected pigs, using tiamulin at 10 mg and 15 mg/kg bodyweight given by injection for three consecutive days, by comparing their weight gains and reduction in lameness scores during a seven day trial period (days 0 to 7) with those of negative untreated controls and positive controls injected with lincomycin at 10 mg/kg bodyweight for three days. Both of the tiamulin treatment levels appeared to be effective, as there were marked improvements in weight gains and reduction in lameness scores in comparison with the negative controls. The improvements were similar to those achieved with the positive control, lincomycin.