In vitro immune responses to Bacillus Caimete Guerin (BCG) vaccination were studied in three month old infants vaccinated at birth which showed positive in: 16) or negative (n: 16) PPD skin reactions and in non vaccinated infants of the same age (n • 15) by specific PPD induced blastogenesis assay and non specific phytohemaglutinin (PHA) stimulation. In non vaccinated infants blastogenic responses (cpm) at PPD doses of 25, 50 and 100/Jg were slight {770 to 1 700 cpm), whereas among vaccinatec PPD positive infants corresponding values were 27925 ± 7 095, 34 297 ±8717 and 45 354 ± 8 453 cpm respectively fp < 0.001). In vaccinated infants showing negative PPD skin reactions, responses were similar to those seen among non vaccinated babies in ten cases, while the other six responded just like vaccinated children with positive PPD reaction (2 744 ± 7 145 cpm), but only under 100 jUg PPD doses (p > 0.05)-Stimulation indexes under PPD or BCG were in all similar. PHA response was 124945 ± 53 200 cpm in non vaccinated infants, 156 263 ± 43 540 in PPD non responder vaccinated subjects and 138 21 7 ± 22345 in PPD-positive vaccinated babies (p > 0-05). Stimulation indexes after supressor adherent cell elimination were lower in all studied groups but reached statistical significance only among vaccinated PPD non responders |p < 0.01). Serum concentration of IgG and IgiV antiPPD antibodies were similar among infants of all three groups and in pooled sera from blood bank's healthy donors, but significantly lower than in serum from patients with active tu bercu lous disease.