Preservative Kathon CG (K-CG) is a commercial preparation, consisting of the two active ingredients (a.i.), 2-methyl-4-isothiazolin-3-one (243-K-CG) and 5-chloro-2-methyl-4-isothiazolin-3-one (5243-K-CG) and also of other components. Both a.i. are known contact sensitizers in humans. In this study guinea pig maximization tests were performed with the a.i. in order to assess and compare the degrees of the sensitizing capacities. The animals were also rechallenged with the sensitizer and 4 chemically related compounds, all being preservatives or known ingredients in preservatives, in order to study the cross-reaction patterns. 5243-K-CG was demonstrated to be a strong sensitizer and 243-K-CG a weak sensitizer. With 5243-K-CG as the sensitizer, 4.5-dichloro-2-methyl-4-isothiazolin-3-one was a possible cross-reacting compound. Possible cross-reactivity was indicated between the a.i. when 243-K-CG was the sensitizer.