Many of the soluble factors controlling growth and differentiation of hematopoietic cells have recently become available as pure and recombinant proteins. This had led to a better understanding of where in the pathway of maturation and how these molecules act, as well as to the identification of specific receptors. As most readers of scientific literature have experienced, the extensive amount of data currently being published on this subject can lead to a great deal of confusion--especially for those scientists not directly involved in the field. Although new data is constantly being generated and future research will further elucidate the complexity of the role of signal molecules for hematopoietic cells, this short article is an attempt to give a simplistic overview of the present status.