A genotype of Trifolium repens L. capable of sustaining high-frequency plant regeneration from long-term (24-month old) cell cultures has been selected. Numerous densely cytoplasmic meristemoids were formed in suspension cultures following the coordinate removal of 2,4-dichlorophenoxyacetic acid (2,4-D) and trichloropicolinic acid (picloram) from the medium and an increase in the NH 4 (+) concentration. Some meristemoids arose from single cells in culture. Increasing the NH 4 (+) concentration in the medium resulted in increased meristemoid formation and decreased the growth rate. Ammonium stimulated meristemoid formation when it was the sole source of nitrogen only if a lethal shift in the pH of the medium was prevented. Meristemoids plated on hormone-free agar medium developed directly into shoots which spontaneously formed roots.