The polymorphism of the second component of human complement was studied by means of isoelectric focusing in polyacrylamide gels with subsequent complement-dependent lysis of sensitized sheep erythrocytes in an agarose overlay containing C2-deficient or normal human serum. In a material of 289 unrelated individuals the following gene frequencies were observed: C2^1 = 0.965 and C2^2=0.035. The rare phenotype C2 2 (=C2 B) could be seen once in a child of a C2 1-2 heterozygous mother. The investigation of the C2/HLA relationship revealed a very close linkage : Among 62 informative meiotic divisions one recombination between HLA-B and C2 was found (i.e. 1.61%); in addition, C2^2 was significantly associated with HLA-B 15 and -Cw3. In a family with an HLA-A/C crossover C2 segregated together with HLA-C, in a family with an HLA-B/D(DR) crossover C2 segregated together with HLA-D(DR). This supports the assumption of a C2 structural locus outside HLA-B, probably near HLA-D(DR).