This study was undertaken to investigate the effect of ketotifen on the immune system of asthmatic children receiving prophylactic treatment with ketotifen. E. rosettes, mitogenic response to phytohemagglutinin (PHA) and concanavalin A (Con A) as well as Con A induced suppression were measured in 15 asthmatic patients before and 1-3 months after treatment with ketotifen (1 mg x 2/day), and in 20 normal healthy controls. No significant difference in immunological parameters studied were found between the pre and post treatment periods in the asthmatic patients, neither was any difference demonstrated between the asthmatic and control group studied.