The tuber reaction to Phytophthora infestans was evaluated in individual genotypes of four diploid and four tetraploid potato families, their parents and standard cultivars. The genotypes were tested several times. Two testing methods were used, based on evaluation of tuber slices or whole tubers, respectively. All inoculations were done with MP 245, a fungus isolate with a broad virulence spectrum. If the reaction of tuber slices was evaluated, the distribution of genotypes in several families deviated considerably from a normal one. In some distributions more than one peak was detectable, indicating segregation of major genes. Genotypes transgressive in resistance were found in four families. If the reaction of whole tubers was evaluated, the distribution of genotypes did not deviate significantly from a normal one. Genotypes transgressive in resistance were found in one family. The tuber reaction to P. infestans depended to a large extent on testing conditions.