Soon after the discovery that plant lectins stimulated the proliferation of human T-lymphocytes, soluble mitogenic factors (MFs) were found in the culture supernatants (Gordon & Maclean 1965, Kasakura & Lowenstein 1965). Since 1965, many MFs have been described, and depending upon the assay used, as many different names and acronyms were applied to the activities. The plethora of these designations, and the lack of a complete understanding of the operational itnportance of the MFs, led to considerable confusion in the field. The major unanswered question concerned the functional significance of MFs; ie were they simply interesting phenomena which accompanied lymphocyte activation, functioning to amplify the proliferative response, or alternatively, were such MFs obligatory and responsible for T-cell proliferation? In 1976, it was reported that conditioned media from lectin-stimulated mononuclear cells contained an MF that would support the continuous exponential proliferative growth of lectin-activated human T-cells (Morgan et al. 1976). This discovery allowed for the construction of a rapid, quantitative assay for the T-cell growth factor (TCGF), in that the growth of the cultured T-cells was entirely dependent upon an exogenous supply of TCGF. Within 24 h, sigmoid TCGF-dependent dose-response curves, which were amenable to probit analysis, yielded reproducable, quantitative data, such that a comparison of TCGF liters was possible (Gillis et al. 1978b). The importance of this assay to the subsequent studies cannot be overemphasized: the lack of a rapid, simple, quantitative assay had greatly impeded progress in other areas of lymphokine research.