The calcium-binding proteins of the S100 family show tissue-specific expression. In this study, the mRNA and protein expression of five S100 calcium-binding proteins was investigated in different guinea-pig smooth muscle preparations. Transcripts of cDNA of S100A1, S100A4, S100A6 and S100A10 were amplified from smooth muscle RNA of neonatal and adult urinary bladder, ileum, aorta and from freshly isolated and cultured urinary bladder smooth muscle cells. S100B was not detectable in smooth muscle RNA, but was seen in control RNA isolated from brain. The pattern of S100 mRNA expression did not change during postnatal development and cell culture of smooth muscle. The structural homology of guinea-pig S100 proteins reverse transcription-polymerase chain reaction (RT-PCR) sequences compared to other species was 85-93% (human), 83-88% (rat) and 81-87% (mouse). Protein expression of S100A4, S100A6 and S100A10 was investigated in aorta, ileum, bladder and cultured bladder smooth muscle cells by Western blot analysis using polyclonal antibodies against guinea-pig-specific S100 immunogenic peptide sequences raised in rabbits. The results show that the proteins S100A4, S100A6 and S100A10 are expressed in the smooth muscle of ileum, bladder and aorta. S100A4 and S100A6 proteins are also expressed in cultured smooth muscle cells. The results of this study suggest that the calcium-binding proteins S100A1, S100A4, S100A6 and S100A10, but not S100B, are expressed in guinea-pig smooth muscle, and could be potentially involved in the regulation of cytoplasmic Ca(2+)-concentration and/or in signal transduction in smooth muscle.