The specificity of the contact sensitivity induced by various chemically related oxazolones was studied. Mice were painted with 1 of them (immunogen). They were divided into several groups and challenged (ear painting on day 5) with various concentrations of either the immunogen or a related oxazolone. The degree of the contact sensitivity was estimated by ear weight or by incorporation of a DNA precursor. Higher concentrations of a challenge compound elicited a stronger DTH reaction than lower concentrations. The efficiency of each compound was defined by the concentration that caused a 30% increase of the ear weight or 150% increase of incorporation. Both represented ca. one-third of the maximal response (1/3 max), and both methods gave similar results. Two types of specificity patterns were observed. One is based on limited data, and was exhibited by the pair furyl Ox and propenyl Ox. Furyl Ox was a more efficient challenge compound than propenyl Ox when mice were primed with furyl Ox. The reverse was true when mice were primed with propenyl Ox. The other pattern was exhibited by the pair propenyl Ox and phenyl Ox. When mice had been primed with phenyl Ox, the 1/3 max response required either 3 mM phenyl Ox or 17 mM propenyl Ox (6-fold difference). The unexpected finding was that phenyl Ox was also a (2.5-fold) more efficient challenge compound for mice that had been primed with propenyl Ox (a heteroclitic contact sensitivity).