pl markers are lyophilized mixtures of stable, salt-free, highly purified proteins. The markers may be run in parallel with the experimental samples on isoelectric focusing gels. These markers are not pre-stained. The precisely defined pI of the markers allows a permanent and accurate calibration of sample components. pH gradients and pI can be measured to within 0.01 to 0.05 pH unit. The progress of the focusing experiment can be monitored visually by the focusing of methyl red or cytochrome c.