AbstractDouble‐label immunofluorescence of tubulin and preicentriolar material (PCM) was carried out with mitotic nuclei in the coenocytic green algaErnodesmis. Spindle poles are heavily labeled with serum 5051 (anti‐PCM) from midprophase through mid‐ to late anaphase, and bright fluorescence is also evident at the tips of the elongated interzonal spindle in telophase nuclei. Very faint labeling with anti‐PCM is also detected throughout the spindle (and/or its matrix) at all mitotic stages. Control treatments demonstrated that nonspecific surface labeling of chloroplasts with anti‐PCM may be due to some naturally occurring component of human sera rather than to specific labeling by the anti‐PCM serum. Ultrastructural work indicates that the centrosome is always associated with spindle poles through anaphase, but not with the tips of the interzonal telophase Immunoper‐oxidase electron microscopy verifies that anti‐PCM labels the centrosomes of mitotic nuclei in these cells. However, labeling is also present inside the presistent nuclear envelope at the spindle poles, during metaphase, anaphase, and at the tips of the interzonal spindles. Regions of heaviest labeling correspond with amorphous material near the centrioles and at the spindle poles, as evident in conventional electron microscope preparations. The origin of intranuclear amorphous material that labels with anti‐PCM is unclear, but the ends of many spindle microtubules are embedded in it, especially at anaphase, and the tips of microtubules near the amorphous material are often labeled with the antiserum. These results indicate for the first time that serum 5051 does indeed label PCM at the poles of centric spindles in plant cells. Although the location of the labeled material suggests it is associated with the nucleation of spindle microtubules, this conclusion requires more information about microtubule dynamics in these cells. Caution is also warranted in interpreting variant anti‐PCM labeling patterns in other plant cells because of spurious labeling of the spindle itself and other cyt
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