This is a continuation of an ongoing study into the function of nuclear and annulate lamellae pores using standard electron microscopic techniques. Our preliminary results let us believe that the pore frequency and distribution are not a result of the physical parameters of the nucleus. By the nature of their location, we speculate that the pore complexes represent sites of control mechanisms. Whatever the functions of annulate lamellae in germ cells and during early embryonic development, it seems to be reexpressed in malignant cells but not in the cells from which they derived. It is therefore a morphological indication of malignancy. The time sequence of appearance of annulate lamellae in the course of experimental transformation of cells will therefore be investigated. Because of the appearance of this structure in embryonic and malignant cells, the elucidation of its functions may be of basic interest. BIBLIOGRAPHIC REFERENCE: Maul, G. G. Fibrils attached to the nuclear pore prevent egress of SV40 particles from the infected nucleus. J. Cell Biol. 70:714-719, 1976.