Abstract.Commercial reagents of non‐human origin were used to provide a room temperature, microtray chromogenic assay for factor VIII. The assay allows determination of factor VIII in the range 15–150% of normal plasma levels with a coefficient of variation of 3–4%. Haemophilic plasmas and factor VIII concentrates may also be assayed by varying the pre‐dilution step. A good correlation was demonstrated with results obtained using an established one‐stage clotting assay (r