Pig Clq, a subcomponent of the first component of complement, was isolated in a fully hemolytically active form using precipitation in the presence of chelating agents at low ionic strength. Three times precipitated Clq was highly purified as shown by immunoelectrophoresis, polyacrylamide gel electrophoresis and ultracentrifugation. The yield of Clq using this method ranged from 40 to 55%. Pig Clq was also purified using precipitation with EDTA followed by affinity chromatography on a pig IgG-Sepharose 4B column. However, overall recovery of Clq was only about 10%. The purified Clq was heat-labile (56 °C, 30 min) both structurally and functionally, had an apparent molecular weight of 400 600 daltons as determined by ultracentrifugation, and restored the hemolytic complement activity of Clq-depleted sera of various species thus confirming interchangeability of this subcomponent. Cross-reactivity of pig, human, guinea-pig, mouse, rat and frog Clq-containing sera with monospecific anti-pig Clq antiserum showed a high degree of antigenic similarity.