Amides of lithocholic acid (3α-hydroxy-5β-cholan-24-oic acid) with 6-aminocaproic acid and 4-aminobutyric acid were prepared and examined by electron impact ionization mass spectrometry. Both these compounds gave an unusual [M − 57]+ fragment. Since the product-ion analysis of [M − 57]+ revealed the presence of fragments corresponding to the intact steroid nucleus in addition to that of the original amino acid (6-aminocaproic acid or 4-aminobutyric acid), we concluded that the integrity of the steroid amide had been retained in this fragment. The absence of this fragment from the product-ion spectrum of [M − CH3]+ rules out the sequential loss from the molecular ion of 15 + 42 u as the origin of this signal. Mass spectrometry of the 24-13C-labelled lithocholylcaproylamide showed the retention of the label in the [M − 57]+ fragment. In contrast, the corresponding compound labelled with deuterium at C(23) showed a significant loss of the label during the formation of this product ion at [M − 58]+. In addition, through a combination of derivatization and tandem mass spectrometry, it was demonstrated that this loss of 57 u represented a rearrangement with the expulsion of a C4H9 radical from the side-chain spanning C(20) to C(23) resulting in a truncated steroid-amide fragment. This fragmentation pattern has not been observed in bile acid conjugates with N-α-amino acids.