Lowest CAS Registry Number on record
I work at CAS and recently answered a similar question on Twitter (@CASChemistry). In fact, there are 8 pre-50-00-0 CAS Registry Numbers in our database:


35-66-5
35-67-6
36-51-1
36-88-4
37-71-8
37-82-1
37-87-6
38-26-6


So, the lowest CAS Registry Number on record is 35-66-5 for "Benzacridine, 7,​9-​dimethyl- (9CI)".