And also accidental , in the sense that Chain only poured all of the penicillin hitherto made by Chain' own methods into two healthy mice in March 1940, out of spite.
Florey had just told him the day earlier, during a very heated argument, that Norman Heatley would be making the crude penicillin from now on and by Heatley's method.
Chain was determined to establish his (or any) penicillin was in fact very non-toxic , though he should have put his brew into infected mice as he then would have really made his mark in history for sure.
But, Chain's spite at least jumped the gun on whatever decade Florey was originally planning to interrupt his precious personal research long enough to test penicillin on infected lab animals. (If that was even ever to come about in the original project -- mice cost money.)
He had at least forced Florey to put penicillin into some infected mice (the next stage in the process) or risk forever losing a claim to be the first to ever do it in history ----- and Florey liked nothing better than getting the glory of doing something first, even if his minions had to do all the hard work forever afterwards.
Much to Florey's surprise, the mice lived !
Florey - to his dying day - hated clinicians and clinical work : hated patients in fact
Now Florey was almost forced to try it on humans in an attempt to save their lives ---- which he did , almost exactly twelve months after the first mouse experiments ----------never one to rush into clinical, live-saving, work was our Dr Florey,MD.....