Suspension cultures were incubated in the presence and absence of gibberellic acid (GA(3)) in an attempt to define a new experimental system for study of the molecular action of gibberellins upon growth. Unlike many suspension cultures, an auxin-independent green clone from spinach (Spinacia oleracea L.) and an auxin-dependent line of "Paul's Scarlet" rose (Rosa sp.) were promoted in expansion growth by GA(3) at 10(-11) to 10(-6) molar. In Rosa the cells also elongated upon GA(3) treatment whereas in Spinacia they remained isodiametric.Attempts were made to intensify the response. The effect of GA(3) in Spinacia was stronger when gas exchange between the culture and the laboratory air was facilitated. The response of Rosa was dependent on the presence of a cytokinin, although this culture did not require exogenous cytokinin for serial subculture. GA(3) antagonized the growth retardant AMO-1618 in Rosa but not in Spinacia. In general, conditions that enhanced growth also rendered GA(3) a more effective promoter; conversely, GA(3) tended to become inhibitory under conditions that permitted only slow growth.