The effect of exogenous growth substances on the yield of seedlings, on some aspects of fruit development, on seed and embryo growth and on germination was studied in two crosses, 4n M. Jewel selfed and 4n M. Jewel x 2n M. Jewel, following treatment of the young fruits with combinations of gibberellic acid (GA), para-chlorophenoxyacetic acid (CPA) and 2-naphthoxyacetic acid (NOA) 2, 4, 6, 8 and 10 days after pollination. The treatments affected the set of large pyrenes, seed size and percentage germination but had no effect on embryo size or on the percentage of pyrenes containing seeds. The number of seedlings obtained was affected both by set and by percentage germination. Seed size was positively correlated with percentage germination in the 4n x 2n cross only. A mixture of CPA and NOA increased seedling yield in both crosses.