In the field of the transplantation of organs, there is a great need for an in vitro test of viability which would confirm that the organ was capable of performing its normal in vivo functions. Such a test should ideally be simple, rapid and reproducible. Preliminary studies using the Haematoxylin-Basic Fuchsin-picric acid (HBFP) staining reaction to assess myocardial ischaemia in resuscitated and preserved hearts would suggest that this test meets many of the requirements of a viability assay. The test has been employed in hearts which have been in a state of anoxic arrest for 30 min and then resuscitated and preserved as an autoperfusing heart-lung preparation. The 'positive' response after 30 min anoxic arrest reverts to a 'negative' response after 2 h myocardial perfusion. In hearts which have been preserved as an autoperfusing heart-lung preparation with no interim period of anoxic arrest the HBFP stain response remains 'negative' throughout, confirming satisfactory myocardial perfusion. © 1977 Chapman and Hall Ltd.