I-309, a chemoattractant for monocytes but not neutrophils, is an approximately 8.5 kDa polypeptide of 73 amino acids. Human I-309 is thought to be a homolog of the mouse TCA3, and shows 42% amino acid homology to mouse TCA3.

I-309, a glycoprotein, is a chemoattractant for monocytes. It is secreted by monocytes, activated macrophages and T lymphocytes. Presence of I-309 in lymph node lymphatic sinuses controls metastases into the lymph nodes. It suppresses CCR8 (C-C motif chemokine receptor 8)-induced HIV (human immunodeficiency virus) infection and also participates in angiogenesis. I-309 is also referred to as CCL1 (C-C motif chemokine ligand 1).

Lyophilized from 30% acetonitrile and 0.1% trifluoroacetic acid containing 0.5 mg bovine serum albumin.

Bioactivity measured by ability of I-309 to chemoattract hCCR8 transfected BaF/3 cells.