A 36-kDa immunosuppressant protein (Da-p36) was isolated from salivary glands of feeding female ixodid ticks Dermacentor andersoni, using its affinity for UltraLink™ Biosupport Medium (Pierce, Rockford, Illinois)/protein complexes. Using a nested set of forward degenerate oligonucleotide primers corresponding to Da-p36 N-terminal amino acids, a cDNA encoding the immunosuppressant protein was isolated by 3′ rapid amplification of cDNA ends. The resulting 772-base pair cDNA encodes a novel protein with predicted molecular weight of 24.9 kDa. Sequence analysis revealed the presence of 5 potential glycosylation sites and 1 myristylation site. Immunoblot analyses showed native Da-p36 is present in salivary glands and saliva from both male and female D. andersoni but not in salivary glands or saliva from Amblyomma americanum or Ixodes scapularis. Reverse transcription polymerase chain reaction and immunoblot analyses showed that Da-p36 expression is temporally regulated in salivary glands with maximum mRNA levels preceding maximum Da-p36 accumulation that occurred at day 6 of feeding. The levels of Da-p36 mRNA and protein were greatly reduced in salivary glands from near-replete females removed from sheep after 8 days of feeding. These data are consistent with a role of Da-p36 in immunosuppression during feeding.