is a genetically engineered variant of human macrophage inflammatory protein-la with improved solution properties. We show here that it mobilizes stem cells into the peripheral blood. We investigated the mobilizing effects of 88-10010 on the numbers of circulating 8-day spleen colony-forming units (CFU-S8), CFU-S,2, and progenitors with marrow repopulating ability (MM). A single subcutaneous dose of 88-10010 caused a twofold increase in circulating numbers of CFU-SI, CFU-S12, and M M 30 minutes after dosing. We also investigated the effects of granulocyte colonystimulating factor (G-CSF) and the combination of G-CSF with BB-10010 on progenitor mobilization. Two days of G-CSF treatment increased circulating CFU-Ss, CFU-SI2, and M M progenitors by 25.7-, 19.8-, and 27.7-fold. A single administration of BB-l0010 after 2 days of G-CSF treatment increased circulating CFU-S8, CFU-SI2, and M M even further to 38-, 33-, and 1 00-fold. Splenectomy resulted in increased T The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ''advertisement'' in accordnnce with 18 U.S.C. section 1734 solely to indicate this fact. Great Britain and British Biotech Pharmaceuticals Ltd. 0 1995 by The American Society of Hematology. 0006-4971/95/8512-0044$3.00/0 3412 MATERIALS AND METHODSMice. Male B6D2F1 mice between 10 and 12 weeks of age were used throughout. They were allowed food and water ad libitum for the duration of the studies and all experimental procedures were