Contact with metal working fluids (MWF) is an important factor in the development of occupational hand dermatitis in patients working in the metal processing industry. Biocides are added to MWF as preservatives and are known sensitizers. We analysed 318 patch test reactions to N,N-methylene-bis-5-methyl-oxazolidine 1% in petrolatum (5-methyloxazolidine, Grotan OX, CAS no. 66204-44-2, a formaldehyde releasing biocide) present in coolant oils in patients exposed to MWF and with suspected occupational dermatitis (OD). Positive allergic reactions were noted in 15 (4.7%) cases. In 7 (47%) cases, we were able to confirm relevance by examining the material safety data sheets and these showed that N,N-methylene-bis-5-methyl-oxazolidine was present in the oils used by the patients. No certain relevance could be proved in 8 cases, but in these instances, further information on the biocides used in the oils could not be obtained. Positive reactions to formaldehyde were seen in 11 (73%) patients. Sensitization either to this biocide and/or formaldehyde allergy was considered to have been likely to have made a contribution to the workers' dermatitis.