Immunoglobulin kappa light chains are predominant in normal human serum and a kappa/lambda ratio of 1.7 to 2 has been reported. However, little is known of the partition of light chains in secretions. The levels of IgA, kappa and lambda were assayed by nephelometry in a series of secretions and in normal human serum. Although kappa chains remained predominant, the different secretions studied could be classified according to their kappa/lambda ratio. In saliva (P < 0.001), nasal fluid (P = 0.01) and tears (P = 0.04) the kappa/lambda ratio was significantly lower than in serum. Conversely, higher kappa/lambda ratios were obtained in gastric juice (P = 0.005) and hepatic bile (P = 0.004) and no significant difference was noted between serum and gall bladder bile (P = 0.62). The lower ratios were all observed in fluids produced by glands surrounded by lymphoid tissue included in the mucosae-associated lymphoid tissue. These data strengthen previous observations that suggest a preferential production of lambda chains in human mucosae.