Abstract Assays for alkaline phosphatase, β-galactosidase, penicillinase and peroxidase were optimised for quantitation in microtitre plate wells. Their value as labels in microtitre plate enzymeimmunoassay (EIA) for progesterone was assessed following coupling with 11α-hydroxyprogesterone 11-glucuronide using an active ester procedure. Bridge-heterologous antiserum (11α-hydroxyprogesterone 11-hemisuccinate-bovine serum albumin as immunogen) was used to minimize bridge recognition. The limits of detection of the enzymes were in the order penicillinase > peroxidase > alkaline phosphatase > β-galactosidase. Under appropriate conditions it was possible to achieve > 50% displacement of label with 50 pg of progesterone for all four labels.