Urinary granulocyte colony stimulating activity (CSA) was studied in normal individulas donating granulocytes. Donors were given corticosteroids 2 h prior to a 4-h leukapheresis using an Aminco celltrifuge in which hydroxyethyl starch was introduced into the donor line. Urine was collected 12-24 h prior to the procedure and 12-24 h beginning at the time of administration of corticosteroids. Colony stimulating activity was measured using mouse marrow cells grown in soft agar. After leukapheresis a significant increase in protein excretion was noted (139.94 +/- 28.1 to 288.69 +/- 63.8 mg per gram of creatinine) and the bulk of the protein was albumin. CSA isolated from G-75 Sephadex columns was increased in five donors, decreased in five donors and undetectable in nine donors. No CSA inhibitors were detectable. There was a significant correlation between the quantity of protein recovered from G-75 Sephadex column and CSA.