Human G myeloma (7S γ 2 -myeloma) proteins were investigated for relationships between Gm and Inv genetic factors and the different antigenic types of heavy polypeptide chains (γ-chains) and light polypeptide chains. Myeloma proteins were isolated from the sera of 1 Chinese, 60 white and 28 Negro individuals. These 89 proteins were tested for eight Gm factors [Gm(a), Gm(x), Gm(b 2 ), Gm(f), Gm(b 1 ), Gm(b 3 ), Gm(b 4 ), and Gm(c)], and two Inv factors [Inv(l) and Inv(b)]. Results of the tests were correlated with the four γ-chain subclasses (γ 2a , γ 2b , γ 2c and γ 2d ) and the two types of light polypeptide chains, κ-chains (type K or I) and λ-chains (type L or II) found in human IgG molecules. 1. Gm factors were limited to myeloma proteins with heavy polypeptide chains of the γ 2b - and γ 2c -subclasses. No Gm factors were detected on γ 2a - and γ 2d -myeloma proteins or on a "heavy-chain" disease protein of subclass γ 2d . 2. γ 2b -Proteins were positive for at least one Gm factor and were either Gm(a+), Gm(a + x+), or Gm(b 2 + f+). 3. γ 2c -Myeloma proteins, and one γ 2c -"heavy-chain" disease protein, were positive for at least one Gm factor and contained various combinations of factors Gm(b 1 ), (b 2 ), (b 4 ), and (c). Myeloma proteins from 3 Negroes were included in this group. 4. Inv factors (l) and (b) were limited to myeloma proteins with κ-light polypeptide chains. These Inv factors were not detected on proteins with λ-light polypeptide chains. 5. Most (70 per cent) of the γ 2b - and γ 2c -proteins with κ-chains were Inv(l+) or Inv(b+). None of the γ 2a - or γ 2d -proteins with κ-chains, however, contained these Inv factors.