A newly developed microassay offers a sensitive method for quantitating egg yolk reactivity in culture supernatants and samples prepared during enzyme purification. Equal volumes of supernatant, saline, egg yolk suspended in saline, and buffer were incubated in microtiter wells at 37-degrees-C, and the resulting turbidity was measured quantitatively with an ELISA reader at 410 nm. The microassay was used to screen culture supernatants from nine clinical isolates of Pseudomonas cepacia, and the results were compared with those obtained when the isolates were screened on egg yolk agar. The microassay was also used to detect egg yolk reactivity in ammonium sulfate-precipitated fractions of the culture supernatant of one strain, Pc224c, and to determine which fraction of egg yolk contained the substrate for the activity.