Proliferation of rheumatoid and control peripheral blood mononuclear cells (PBMC) to an antigenic acetone-precipitable extract from mycobacterium tuberculosis (MTa) was investigated. Cells were also stimulated with the recall antigen tuberculin PPD (purified protein derivative) and the mitogen OKT3. Controls had a significantly higher response to both MTa and tuberculin PPD than RA patients. However, lymphocytes from patients who had had their disease for 3-10 years proliferated more vigorously to MTa than did PBMD from patients with longer disease duration. There was no difference in the proliferation to OKT3 between patients and controls. We were not able to confirm a previously found correlation between the HLA-DR4 phenotype and lymphocyte transformation to MTa.