Crystal substances being then regarded as 100% pure.
But after he learned - probably sometime in November 1940 - that he had definitely upstaged in October 1940 by Martin Henry Dawson injecting patients after only 5 weeks of work on penicillin, Florey definitely had to lower his standards .
Dawson pushed Florey (and the NAS/OSRD/MRC triad) into PLAN B to try and catchup
So in early 1941, three years after he began work on penicillin, Florey also began, reluctantly, injected patients with impure penicillin of about the same strength as Dawson's penicillin, which is to say not very pure.
Pure penicillin has 1670 units of strength per mg : Florey's was between 20 and 50 units per ml , ie about 2% pure and 98% impurities.
Dawson hadn't worried much about purity, regarding food as perfectly safe even though it is often only a few percent pure protein by weight : as the hungry man once said, as long as impurities were relatively harmless, bring 'em on, pizza boy ....