Previously we show that immune response against parasite and protein antigens differs quantitatively and qualitatively between young and aged mice (Immunology Letter. 40:243-250; 1994; Medicina 58,5/2:617,1998). When young (3 month-old) and aged (12-18 month-old) BALB/c mice were immunized s.c. with 60 mg of OVA plus Bordetella pertussis as adjuvant on day 0, 15 and 30, we found a IgG2a/IgG1 ratio of 0.32 + 0.10 in aged and 0.62 + 0.03 in young mice (p:0.024) and the specific T cell proliferative response (mitotic index) was 2.2 and 5.9 respectively. When the mice were immunized s.c. on days 0, 15 and 30 with 60 mg OVA emulsified in CFA we found that the same parameters mencioned above were similar in aged and young animals. This means that to find in aged mice a similar response to young animals we need an adjuvant that induce a Th1 response. Therefore, we evaluated oligodeoxinucleotides containing unmethylated CpG motifs (CpG-ODN 1826) as an adjuvant of particular interest in aged mice. With this objetive, on day 0 and 15, young and aged mice (n:6) received s.c. 60 mg OVA plus 50 mg of the indicated CpG-ODN. Eleven days after booster both experimental groups show similar specific IgG levels, the IgG2a/IgG1 ratio was 0.78 + 0.04 in 3 month and 0.66 + 0.09 in 12-18 month and the mitotic index of T cells was 6.3 and 6.1 respectively. These results suggest that CpG-ODN could be able to induce a young-like specific immune responses in aged mice. Consequently these findings have important implications for the development of vaccines in aged systems. (Grants from CONICET. CONICOR. SeCyt UNC and ROEMMERS)