Immune functions, including cell surface markers, interleukin-2 receptor levels and responses of lymphocytes to mitogenic stimulation were evaluated in seven growth hormone deficient children ages 4-15 years, during treatment with biosynthetically derived human growth hormone. Treatment resulted in a decrease in % B cells and in % T total cells and also decreases in most individual patients' mitogen responses and interleukin-2 receptor levels. Most of the changes noted were transient and similar to those previously demonstrated during pituitary-derived human growth hormone treatment. Although not resulting in overt clinical manifestations in our patients, we think that potential interactions between growth hormone and immune functions need to be considered by physicians treating children with growth hormone.