When a catalog says "complement", it means serum that was handled so the complement proteins are still active. The blood was drawn cold, the serum separated quickly, and the product frozen within hours and never allowed to warm. Whether that serum will work in your assay comes down to two things: which species it came from and what was done to it afterwards.
Species
The complement cascade is conserved across mammals. Lytic strength and background antibodies are not. For a functional antibody assay you need a complement source that lyses the target efficiently and carries no antibodies of its own against it, otherwise the serum's pre-existing immunity kills the bacteria whether or not the antibody under test does anything.
Baby rabbit complement, from animals 3 to 4 weeks old, is the World Health Organization's recommended source for serum bactericidal and opsonophagocytic assays. Young rabbits have very high lytic activity and have not yet picked up antibodies to the human pathogens those assays target. We wrote more about that in baby rabbit complement, the gold standard for functional antibody assays.
Adult rabbit complement is the traditional source for complement-dependent cytotoxicity and cell-depletion work, where you want lytic strength and background antibody matters less.
Guinea pig complement is the classical reagent for hemolytic assays and complement fixation, and a lot of older protocols still specify it.
Human complement, pooled or single donor, is the right source when the question is how a human immune system would respond. For vaccine work it is used after the IgG and IgM have been removed, so the only antibody in the reaction is the one you added.
What the lot tells you
The hemolytic titer (CH50) is the functional activity of the lot, measured before release. Compare it between lots and use it to set your working dilution. The assay itself is covered in complement activity explained.
Format matters for logistics more than for chemistry. Frozen liquid complement has the highest activity, but it has to stay at −70 °C or colder and can only be thawed once. Lyophilized complement is stored refrigerated and reconstituted at the bench, which makes it the sensible choice for kits and for sites without an ultra-low freezer.
Processing shows up in the product name. Sterile-filtered lots are for cell-based assays and anything that will sit in an incubator for hours; non-sterile lots are fine for a short hemolytic assay. Adsorbed complement has had cross-reactive antibodies against a particular cell type removed, while unadsorbed is the native serum.
Human-derived complement is collected under FDA-licensed conditions from donors tested for HBsAg, HCV and HIV.
Handling
Thaw frozen complement quickly in a 37 °C bath, only until the last ice is gone, then put it on ice. Dilute it in cold buffer that contains calcium and magnesium, and use it the same day. Don't refreeze a thawed vial; aliquot it the first time instead. Keep EDTA and citrate out of the assay buffer, since both chelate the divalent cations the cascade needs.
Picking a source for a new assay
Start with the species the published protocol used, because changing it means re-validating. Then pick the format your logistics can support and the titer your target needs. Our rabbit complement comes from our own colony in Arkansas and our human complement from licensed collection, both in large lots, so a multi-year study can stay on one lot from start to finish. If you're not sure which source fits, tell us about the assay and we'll help you choose.