1. The effect of ampicillin, cloxacillin, flucloxacillin and sulphafurazole on bilirubin on bilirubin binding by pooled human umbilical cord serum and bovine serum albumin was studied in vitro using Sephadex gel filtration. 2. Sulphafurazole displaced bilirubin from binding; both cloxacillins displaced bilirubin from pooled cord serum but not bovine serum albumin. 4. No displacement of bilirubin by the cloxacillins from pooled cord serum could be detected at therapeutic plasma concentrations of these drugs. 5. Scatchard analysis of the interactions showed that displacement of bilirubin by these drugs occurred principally at the primary, high affinity, low capacity binding site.