Unlike antibodies produced in rabbits and other animals, mouse antibodies have proved to be difficult to covalently label with fluorochromes. By careful control of the labelling conditions, a method has been established for labelling mouse Thy-1 antibodies obtained by immunisation with normal thymus cells. The resultant FITC Thy-1 antibodies suffer little loss in cytotoxic properties, react with the correct specificity and can be used to label Thy-1 positive cells.