The holding of a symposium concerned with blood profiles was a success and this can be attributed to the degree of frankness with which people have disclosed their hitherto unpublished data and to the valuable discussion which ranged across species and disciplines. Occasionally people have tended to accept the validity of their own data and to question the validity of data from others which apparently leads to conflicting conclusions. In these circumstances it would have been more constructive to attempt to explain why research groups who were studying similar phenomena should obtain dissimilar results.