The standard synthetic culture medium (Stuart and Street, 1969) has been modified by adjustment of its initial pH to 6.4 and by the addition of gibberellic acid (0.25 mg/l) and of a mixture of 15 L-amino acids formulated from an analysis of the conditioned medium. The minimum effective density for the growth of sycamore cell suspensions in the standard medium is 9–15 × 103 cells ml−1, for the modified synthetic medium it is 2.0 × 103 cells ml−1, and for conditioned medium 1.0–1.25 × 103 cells ml−1. Using either conditioned medium (Stuart and Street, 1969) or the modified synthetic medium it is demonstrated that the growth of cultures initiated at low density is enhanced by a volatile factor released from actively growing cell suspensions. In presence of conditioned medium and this volatile factor cultures can be established from stationary-phase cells at a density of 6 × 102 cells ml−1. The volatile factor can be absorbed in 40 per cent w/v KOH but attempts to replace the factor by air containing carbon dioxide at concentrations up to 5 per cent have so far been unsuccessful.