Five argyrophil, non-argentaffin classical carcinoids of the appendix were found in 19 appendiceal classical carcinoids and were investigated histochemically, immunohistochemically and ultrastructurally. All tumors consisted entirely of argyrophil cells. Three of the five carcinoids were composed almost totally of peptide YY cells and were negative for serotonin. One of them consisted of peptide YY cells (60%), somatostatin cells (40%), and a few cells with glucagon-like immunoreactivity (GLI). The remaining one without peptides was homogeneously immunoreactive for serotonin alone. Ultrastructurally, each of the four peptide-positive carcinoids was composed of one kind of endocrine cell type with round secretory granules. Average diameter of granules were 150, 160, 190, and 210 nm, respectively. The non-argentaffin, serotonin-positive carcinoid showed predominant round secretory granules and a few irregular ones, both being 150 nm in largest diameter. It is suggested that the argyrophil, non-argentaffin carcinoids of the appendix are subdivided into two groups; carcinoids composed mainly of peptide (especially, peptide YY)-positive cells with round granules of D1 and/or L cell type and those of serotonin-positive cells with pleomorphic granules of ECn cell type.