Serum from 36 intravenous drug abusers without acquired immunodeficiency syndrome (AIDS) or AIDS-related complex were tested for concentrations of neopterin and beta 2-microglobulin. The seroprevalence of human immunodeficiency virus (HIV) antibody in this group was 50%. Previous studies of this group showed that the HIV antibody positive patients had significant increases in HLA-DR expression on peripheral blood lymphocytes and increases in serum soluble CD8 antigen. Both neopterin and beta 2-microglobulin concentrations were significantly higher in the HIV antibody seropositive patients compared to the seronegative patients (p = 0.001 and p = 0.005, respectively). A highly significant positive correlation between neopterin and beta 2-microglobulin was found for the seropositive patients (r = 0.8879, p less than 0.0001) as well as for the entire group (r = 0.6054, p = 0.0002). Significant positive correlations were also found between neopterin or beta 2-microglobulin and the percent DR + T cells and CD8 antigen levels, although these correlations were not as significant as that observed between neopterin and beta 2-microglobulin. No relationships were found between neopterin or beta 2-microglobulin and total CD4 cell concentrations or CD4/CD8 ratios. These data demonstrate the significant interrelationships between various immune activation markers in a population at risk for developing AIDS.