To verify this assumption we evaluated another kit, the “/3hCG MAIA-clone immunoradiometric assay” (Serono), comparing the effects of lipemia on the results obtained with it and with Clinical Assays’ monoclonal antibody kits; we used sera from seven male patients, selected for hipemia. We also examined the effects of free fatty acids at concentrations of 1 g/L in hCG-free serum. Both assays were standardized against the First International Reference Preparation (1st IRP). Our results, tabulated below, indicate that triglyceride in excess of 6 mmol/L interferes with the Clinical Assays kit, but does not significantly affect results with the Serono kit. Free fatty acids may also interfere.