Following a previous note dealing with rickets, intestinal malabsorption, virus hepatitis and mongolism, the present paper deals with serum immunoglobulin behaviour in thymomegaly, nephrosis and response to TAB vaccination. Thymomegaly was accompanied by increases in all three Ig classes, followed by normalisation of values shortly after steroid treatment. in nephrotic proteinuria, determination of glomerular selectivity via the evaluation of IgG and transferrin clearance was of assistance with respect to prognosis and therapy. Artificial antigen stimulation of healthy and mongoloid subjects with TAB vaccine was not followed by differences in Ig class behaviour; similar stimulation of full-term and immature neonates, however, showed a rapid and frank increase in immature IgM values with respect to the non-stimulated immature subject of equal age and weight.