It is generally agreed that intra- and interlaboratory variations in the evaluation of eye irritants are mainly due to the subjective judgement of eye lesions. The scoring of the palpebral conjunctivae is a typical example. To eliminate these difficulties in assessing eye irritants the relationship between corneal swelling, which can be determined using an objective technique, and the reading of other symptoms was investigated. The in vivo eye irritation assay was carried out on 34 substances according to the protocol laid down in Annex V, part B of Directive 79/831/EEC of the European Community on the classification, labelling and packaging of dangerous substances. Results were obtained for erythema, oedema, corneal opacity and corneal swelling. Erythema, oedema and opacity were evaluated according to the interpretation rules laid down in Annex VI, part IID of the EEC Directive 83/467/EEC. Corneal swelling was determined by the ultrasonic pachometer technique. Good correlations were found between the mean percentage corneal swelling after 24, 48 and 72 hr and the mean corneal opacity (r = 0.94) and erythema scores (r = 0.93) after the same observation times. The corneal swelling scores measured after 24 hr using the ultrasonic pachometer are comparable with those reported in the literature, which were obtained by optical pachometry. The good correlation with the other effects scored and the good repeatability of the results opens the possibility of introducing an objective and sensitive method into the ocular irritation assay and so reducing intra- and interlaboratory variations.