A convenient method for infecting Brassica seedlings in liquid medium with the Phytopathogenic fungus Plasmodiophora brassicae was developed. The infection rates of two susceptible and two tolerant Chinese cabbage varieties with wild type P. brassicae were investigated. The content of free indole‐3‐acetic acid (IAA) was determined using combined gas chromatography‐mass spectrometry (GC‐MS) with [¹³C₆]‐IAA as internal standard during the first period of infection with the fungus (5–14 d) and compared with the indole glucosinolate content of two Chinese cabbage varieties (one tolerant, one susceptible) in the same developmental stages, The results showed that the mean of the IAA content in the infected plants was approximately 66.5% higher than that of the non‐infected controls. In both the susceptible and tolerant varieties higher levels of IAA were found in the infected plants 10 d after the beginning of incubation. After 14 d of incubation IAA levels decreased in the susceptible, infected plants and increased in the tolerant, infected plants.