Five gram-positive bacterial strains were selected for absorption studies of human serum samples. Strain AR1 (group A, M-type 1) and G148 (group G), with strong immunoglobulin G (IgG) binding capacities, and strain AW43 (group A, M-type 60), binding both IgA1 and IgA2, were compared with Staphylococcus aureus Cowan I and with Staphylococcus epidermidis L603. Both AR1 and G148 were capable of completely absorbing out serum IgG. In contrast, S. aureus Cowan I left a fraction unabsorbed, as expected from its known lack of IgG3 binding. Strain AW43 absorbed out all serum IgA, using a 10-microliter bacterial pellet for 20 microliter of serum. Serum IgM levels were slightly reduced by S. aureus Cowan I absorption. On the basis of the experiments, a bacterial mixture was designed consisting of S. aureus Cowan I and group A streptococcus strains AR1 and AW43, with absorption characteristics suitable for use in discriminating between early IgM and late IgG and IgA immune responses in routine serological work. A new type of bacteria-mammalian protein binding was discovered. Human serum albumin was completely absorbed out by strain G148 and to a lesser extent by strain AR1 and AW43. S. aureus Cowan I and S. epidermidis were negative. The binding capacity of G148 for albumin equalled that of Cowan I for IgG. The binding pattern of albumin to the strains was different from those of IgG, IgA, IgM, fibrinogen, haptoglobin, or aggregated beta 2-microglobulin and therefore seems to represent another type of bacterial-mammalian interaction with a specific albumin receptor on the surface of streptococci.