- Measurements of cellular immunity including the percentage of T-cell mitogenic response of lymphocytes to phytohemagglutinin (PHA) and concanavalin A (Con A), as well as Con A-induced suppressor function, were determined in 20 asthmatic children (mean age 12.4 +/- 0.6 years) before and after 6 months of theophylline therapy. Only 9 of the 20 children patients that commenced the study completed the entire 6 months of therapy. A group of 37 healthy children (mean age 12.6 +/- 0.3 years) served as control. During the 6-month period of therapy, theophylline levels were maintained between 10 and 20 micrograms%. No significant difference in any of the parameters examined were found between the initial 20 asthmatic patients and the control group. Furthermore, in the 9 patients that were evaluated before and after 6 months of theophylline therapy no significant change in lymphocyte function including suppressor function was detected. We conclude that the beneficial effect of long-term theophylline therapy in asthmatic children is probably not related to an immuno-modulatory effect.