During differentiation and maturation well defined genetic rearrangements of B- and T-cell receptors are followed by a stepwise expression of defined phenotypic properties. Transformational events on such defined steps of cell differentiation result in different types of malignant lymphomas, some of which have these events during early steps of cell differentiation but show further maturation or activation. This is shown for the plasmacytoma but may also be true for Hodgkins disease or large cell anaplastic lymphoma. Inducing signals for maturation, differentiation, and activation are given by cytokines. The loss of a regular function of these cytokines may result in para- or autocrine growth signals in malignant lymphomas.