Immunoassay methods and apparatus are provided which utilize flow cytometry, coated latex microspheres, and fluorochrome labeled antibodies, to simultaneously detect the presence and amount of one or more analytes in a sample. Beads of several different sizes, colors or shapes, each bead are coated with a different analyte, for the simultaneous detection of one or more analytes and of cell components. The invention is also directed to platelet Ig positive control reagents and assays which provide for the setting of the fluorescence positive region for each patient. The platelet control is sized to fit between the platelets and red cells and thus making it ideal as a true biological control.