A polyclonal, enzyme-linked immunosorbent assay (ELISA) of Phytophthora infestans has been developed for use in the determination of fungal biomass during the early stages of infection of tuber discs of Solanum tuberosum. By optimizing the dilution of sample extracts and the dilution of primary anti-P. infestans antiserum, quantification of the biomass of P. infestans in zoosporeinoculated tuber discs could be achieved by 8-18 h after inoculation. Differences in growth between avirulent and virulent isolates of P. infestans on the resistant potato cv. Kennebec were quantified by 32-48 h after inoculation. Together with a comparison of the growth of the same isolates on the susceptible cv. King Edward, these results comprised an ELISA of the Quadratic Check. On the resistant host, the growth of the avirulent isolate was essentially arrested by 16 h after inoculation, whereas that of the virulent isolate continued throughout the time course. On the susceptible host, however, the avirulent isolate appeared more aggressive than the virulent isolate. These results demonstrate that the ELISA, which is often simpler to perform than other procedures for estimating fungal growth, may be used to complement biochemical studies of rapidly induced plant defence responses.