Mesophyll protoplasts were isolated from five pea (Pisum sativum) varieties and cultured in medium 8p, modified to test the effects of varieties, leaf ages, protoplast population densities, pH, and plant hormones on relative division frequency. Protoplasts isolated from the variety Rondo had a much greater capacity for division than those from Century, Laxton's Progress, Trapper, or Afghanistan 72-HI-2. The highest division frequency occurred in protoplasts isolated from the top expanded leaf of seedlings at the three-, four-, or five-leaf stage. The maximum frequency of division was obtained using a protoplast density of 5 × 104to 1 × 105 per millilitre and the 8p medium at pH 5.6 with 850 mg/L of CaCl2∙2H2O, 0.25–1.00 mg/L of 2, 4-D, and 2 mg/L of zeatin riboside. Gradually reducing the osmolality from 510 mosmol to 420 mosmol was essential to achieve a high frequency of cell division.