I have read with great interest the considered article by Dr, Barry Rittman (Microscopy Today#98-8, October, pp 8-9). In describing the difficulties of quantitation in immunohistochemistry, Dr. Rittman has gone to great lengths to give an authoritatively balanced view. It is, therefore, with some trepidation that I raise a few issues which may remain unresolved.

Dr. Rittman provides a list of criteria to be considered before applying quantitative analysis to this method. I would place the question of relating color developed to the amount of substrate present higher than number 7 on the list. To the best of my knowledge, there is little evidence that immunocytochemistry is stoichiometric, i.e.,that there is a relationship between color developed and amount of substrate present, indeed many consider that it is not, which represents a serious drawback.