The study involved 120 cats, of which 60 were treated with Baypamun HK and 60 cats received a preparation of virus free cell culture medium. Therapeutic efficacy was determined by monitoring the general status of health, body weight, skin, lymph nodes, oral cavity, the presence of the p27 antigen and the FeLV p27 antigen concentration in serum. No statistically significant differences between both groups could be demonstrated neither for clinical nor for virologic parameters. Remission of the viremia occurred in 11.7% of the cats treated with Baypamun HK and in 6.7% of the cats treated with placebo. The FeLV p27 antigen level decreased by an average of 7.2% with Baypamun HK and by an average of 5.1% with placebo.