We have used antibodies against different proteins of the nuclear pore complex to identify cytoplasmic annulate lamellae by immunocytochemistry at the light- and electron-microscopic level in various lines of cultured mammalian cells. Although annulate lamellae (AL) are seldom observed in some cell lines, they occur in large numbers in other lineages, also with cell-type-specific size distributions. AL are especially abundant and prominent in African green monkey kidney epithelial cells of line RC37 and bovine mammary gland epithelial, cells of line BMGE. We have studied the distribution of AL in relation to other organelles by using double-label immunofluorescence microscopy and have demonstrated a significant relationship between AL and rough endoplasmic reticulum. We have further shown that AL are disassembled during prophase and reassembled at the end of mitosis, almost concomitantly with the disassembly and reassembly of the nuclear envelope. Cultured mammalian cells that are rich in AL can therefore be used as suitable models for studies of the biogenesis of these lamellae, which can conveniently be detected by immunofluorescence microscopy.