To test the hypothesis that odorant quality perception is altered in olfactory marker protein (OMP)-null mice, we trained and tested adult OMP-null and control mice, using a 5-odorant identification confusion matrix task (animal odorant confusion matrix [AOCM]). On average, control and null mice performed the task at equivalent levels. The composite 5 x 5 response matrix from 40 testing sessions for each subject (both OMP-null and control) was compared with that of every other subject, yielding a dissimilarity matrix of AOCM responses. A multidimensional scaling (MDS) analysis of the dissimilarity data yielded a 4-dimensional solution, with each mouse occupying a point in MDS animal space. Statistical analysis demonstrated significant effects of genotype in determining the location of a mouse in the MDS space. These data suggest, therefore, that compared with that of controls, odorant quality perception is altered in the OMP-null mouse.