3T3-L1-adipocytes produce the adipocyte complement related protein of 30 kD (Acrp3O), which is also designated as AdipoQ. In order to study the expression and secretion of the human homologue of this protein, apMl (adipose Most abundant gene transcript 1, also named gelatin-binding protein of 28 kD [GBP28] or adiponectin), a polyclonal antibody was produced. Both expression and secretion can be detected beginning with day 4 after induction of differentiation. The amount of expressed apMl correlates with the specific activity of the dif-ferentiation marker glycerol-3-phosphate dehydrogenase. Secretion of apMl is increased by the addition of ionomycin. Both the nonhydrolysable dibutyryl-cycloAMP and tumour necrosis factor alpha reduce the expression and secretion of apMl.