1. Cyanide hemochromogen probably contains one cyanide group per heme group. 2. The equilibrium between pyridine hemochromogen and its components, pyridine and reduced heme, is complicated to an unknown extent by the precipitation of reduced heme and the aggregation of pyridine hemochromogen. 3. These complications were not taken into account in R. Hill's experiments on pyridine hemochromogen. 4. Even if Hill's experiments are sound they do not prove his conclusion that pyridine hemochromogen contains two pyridine groups per heme group.