The two mutants of the moss Funaria hygrometrica (auxin-sensitive and auxin-insensitive), isolated after UV irradiation of protoplasts derived from the wild strain protonema cultures, have been biochemically characterized and found to exhibit variations in the endogenous free-pool levels of tryptophan (a precursor for IAA biosynthesis), IAA and cAMP. Both of the mutants have a several-fold higher tryptophan content compared with the wild strain protonema. Whereas mutant 87.13 (auxin-sensitive) is deficient in free-pool IAA, the auxin-insensitive mutant (86.1) accumulates several-fold higher levels of endogenous IAA compared with the wild strain. The two mutants (87.13 and 86.1) also differ considerably in their cAMP content in comparison to wild type. IAA (10 −5 mol/L) marginally lowers the cAMP content in the wild type protonema whereas it significandy lowers the cAMP content in the two mutants. The fact that the auxin-insensitive mutant (86.1) remains as chloronema despite a drastic lowering of endogenous cAMP content by IAA treatment suggests that this mutant (86.1) represents an element in the signal transduction pathway elicited by IAA or IAA and cAMP, leading eventually to cellular differentiation in moss protonema. However, the possibility that some, as yet unknown, biochemical factor(s), in addition to IAA and cAMP, play(s) a regulatory role in protonema differentiation also deserves consideration.