cDNA encoding alpha 1-microglobulin/bikunin (AMBP) was amplified from guinea pig (Cavia porcellus) liver mRNA by reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends methods, cloned and sequenced. The deduced amino acid sequence was found to be homologous to the sequence of AMBP of other mammals (69-76% amino acid identity). It has two Kunitz-type trypsin inhibitor domains in the bikunin part as reactive sites, one in the N-terminal region and another in the C-terminal region. The N-terminal inhibitor domain sequence is well-conserved, but the P1 residue of the C-terminal inhibitor domain sequence was found to be Gln rather than Arg, a residue highly conserved in the AMBP of seven other mammals examined to date. By RT-PCR and nested PCR, AMBP mRNA was detected not only in liver tissue, previously known to be a site of its synthesis, but also in pancreas, stomach, small intestine, colon, lung, spleen, kidney, testis, skeletal muscle, and leukocytes, but not in brain or heart. We examined the AMBP mRNA levels in guinea pig liver by RT-PCR, comparing normal levels and those in a state of inflammation. The mRNA levels, however, did not significantly change.