Human T lymphocytes carry a membrane receptor for sheep erythrocytes (E) related to the CD2 molecule. The E-receptor is found in a soluble form (Rs) in serum and can be quantitated by "rocket electrophoresis" using an anti-Rs serum obtained by immunizing sheep with autologous erythrocytes coated with Rs. Increased serum levels of Rs are found in patients with diseases associated with immunodepression. In the present study, 14 asymptomatic HIV-1 seropositive individuals were investigated regarding their Rs levels and delayed hypersensitivity skin tests every 3 months for a period of 35 months. All these patients progressed to AIDS in this period. Rs serum lev-els have also been quantitated in 14 normal individuals. The mean Rs values in normal individuals, asymptomatic, and AIDS patients were, respectively: 4.8 ± 1.5 mm (SD), 9.6 ± 1.9 mm (SD) and 11.3 ± 2.4 mm (SD). An increase of Rs serum levels was observed when we compared normal individuals with CDC-II and CDC-IV clinical stage patients (P < 0.05, Mann-Whitney test) and CDC-II and CDC-IV patients, (P < 0.05, Wilcoxon test). We have observed a depressed delayed hypersensitivity response to ubiquitous antigens in CDC-IV patients. Our results indicate that Rs serum levels can be used as a progression marker in HIV infected patients.