A solution containing beads or particles with known optical and physical characteristics (e.g., antibodies conjugated with fluorescent dyes, specified size or other label) intended to be used to verify and/or optimize the performance of a flow cytometer or similar single/multichannel instrument. Verification/optimization functions may include optical density (absorbance), fluorescent or luminescent accuracy and compensation, linearity, repeatability, sensitivity, limit of detection and/or light scatter readings at specified wavelengths, and the ability to verify optical alignment and precision of the instrument or analyser. Solutions are typically single-use.