A total synthesis of polyamines was developed using potassium phthalimide as a sole nitrogen source. ¹⁵N-Enriched phthalimide was prepared almost quantitatively from ¹⁵N-enriched ammonium sulfate by a modified method. Key compounds for the synthesis of ¹⁵N-enriched spermidine and spermine were ¹⁵N-enriched putrescine (I), N-(3-bromopropyl)-phthalimide (II), N-(4-bromobutyl) phthalimide (III), and benzylamine, which were easily prepared in high yields from potassium ¹⁵N-enriched phthalimide. Spermidine was synthesized by two alternative methods involving an alkylation of monobenzyloxycarbonyl putrescine with II and a stepwise alkylation of benzylamine with II and III in the presence of KF-Celite. The lattter method allowed the preparation of all seven kinds of various ¹⁵N-enriched spermidines with combinations of the three reagents containing ¹⁵N or ¹⁴N. Spermine was similarly synthesized by an alkylation of N, N'-dibenzylputrescine with II. In these methods, the alkylation using KF-Celite was extremely useful for the synthesis of spermidine and spermine. The present methods were also used to synthesize various other polyamines in high yields.