Hemocytes from crayfish were labeled immunocytochemically for glutamine synthetase. A. GS+ cells (blue) were found in a small percentage (1-3%) of crayfish hemocyte samples (n = 24 smears from 12 crayfish). B. A no-primary control showed that the antibody was specific for GS (no non-specific labeling in the blue channel, Bii), although many cells were labeled with propidium iodide (Bi). C, D. Higher magnification images reveal some cells that have a high cytoplasmic to nuclear ratio; other GS+ cells had one or more pseudopodia (D, arrow). There are three categories of circulating cells in crayfish based on morphological criteria , two of which can be distinguished in Ei. The image in Ei shows the outline of the cytoplasm and nucleus of a GS+ cell (arrowhead), which appears to be of the semi-granular variety. A large granular hemocyte (white arrow) with large refractive granules is also in the field of view. The fluorescent channel alone (Eii, arrowhead) and Eiii (composite: Ei plus Eii) reveal one labeled GS+ hemocyte. Hemocytes were counter-stained with propidium iodide (red) and were imaged using confocal microscopy (A-D) and Nomarski optics (Ei, Eiii). Scale bars: A, 50 μm; B, 100 μm; others, 10 μm.