Retinoic acid and analogues (retinoids) are able to induce the differentiation of F9 murine embryonal carcinoma stem cells into endoderm-like cells. The secretion of plasminogen activator (PA) which accompanies this differentiation is a good index of the biological response of F9 cells to retinoids. We have previously reported that the potency of a series of natural and synthetic retinoids, evaluated by the concentration which provokes half-maximal induction of PA, correlates well with the affinity of these compounds for the endogenous F9 nuclear retinoic acid receptors, but not for the cytosolic retinoic acid binding protein, CRABP. In this paper we show that various retinoids differ, not only in terms of potency, i.e. the dilution at which they are active, but also in terms of the amount of PA that they induce. This parameter, called amplitude, is used to quantify the extent of PA induction by a given retinoid relative to retinoic acid. The amplitude parameters of synthetic retinoids are found to vary over a wide range and are independent of both potency and binding affinity for F9 retinoic acid receptors. It is proposed that the amplitude of the biological response to a given retinoid is the resultant of three factors: (i) the total or partial agonist character of the retinoid; (ii) the binding spectrum of the retinoid for the various types of retinoic acid receptors; (iii) the chemical and metabolic stability of the retinoid in the test system.