Pasture bloat is a serious economic and animal welfare problem in cattle grazed on legumes in New Zealand. Analysis of salivary proteins from dairy cattle in herds bred for either low or high susceptibility to bloat has resulted in the identification of a 30 kilodalton protein, which we term bSP30, whose relative abundance is negatively correlated with bloat score (r = -0.40 +/- 0.12). From 74 animals sampled, relative abundance of bSP30 was 66 +/- 15% higher in the low-susceptibility herd than in the high-susceptibility herd. Relative abundance of bSP30 also varied significantly within individuals, according to feeding or time of day, and from day to day. A sequence homology search of 38 amino acids derived from three tryptic fragments of the protein suggests that the amino acid sequence of bSP30 has not been described previously. Amino acid analysis indicates that bSP30 is not a member of the proline-rich family of salivary proteins. The function of bSP30 is unknown but it is conceivable that it plays a role in the aetiology of bloat.