This study cloned and sequenced equine transforming growth factor (TGF)-beta1, yielding a unique nucleotide structure which predicted amino acid substitutions not seen in other mammalian species. The nucleotide sequence homology was 89% to bovine, 91% to man, 90% to ovine, and 86% to rat. Derived amino acid sequence comparison showed that the equine protein was unique, differing by two residues from man, cow, sheep, pig, and dog, and by three residues in the rat. Subsequent use of the cDNA clones to examine the expression of the TGF-beta1 gene in various tissues indicated predominant expression in adult spleen and kidney, with an age-related peak in cartilage expression at 12 months, followed by a decline as the animals matured. Northern blots showed that the predominant transcript sizes were 2.5 and 1.9 kb. More sensitive mRNA detection using PCR reaction showed peak cartilage TGF-beta mRNA levels in horses 0.7 and 1 year of age, with declining expression in older animals (2.5 and 5.5 years of age). In conclusion, although the primary nucleotide sequence of equine TGF-beta was relatively homologous to that of other species, the resulting amino acid sequence was unique to the horse, differing by two residues from the majority of mammalian sequences, where the peptide structure is identical. Expression of TGF-beta was particularly evident in spleen and kidney, and showed an age-related increase in expression in cartilage as the animals approached maturity and then a decline with progressive aging.