T-cells develop as a consequence of intrathymic and postthymic events, in which hemopoietic progenitors are differentiated into precursors and effector cells. We are proposing that such process includes three integrated steps: (1) T-cell differentiation; (2) selection of the T-cell repertoire and (3) specialization into functional subsets of T-cells. Although there is evidence of specific and nonspecific humoral factors (i.e. thymic extracts, etc.) affecting T-cell differentiation, it is also proposed that the most critical component in this integrated process is the consequence of direct cell to cell interactions between precursor and inducer cells. Thus, the three processes are the consequence of the appropriate or condordant matching of precursor-inducer populations, both at intra- and extrathymic sites. It is also proposed that MHC determinants are critical in permitting the appropriate matching. The model can thus account for nonfunctional differentiation when the appropraite matching is not available and with intrathymic selection by excess cell production favoring the appropriate matching.