Predictions on the toxicity of organic fluorine compounds, based on chemical structural elements, can be rather unreliable. As a matter of fact, closely related substances may possess strikingly different toxic properties. On the grounds of working safety with new chemicals, we were interested in setting up a quick and simple pre-screening test using NRMI-mice which affords preliminary data on acute inhalation toxicities of volatile fluorine compounds. We now report on experimental procedure, evaluation and results of such tests and - a point of much importance - the correlation between the pre-screening data and those obtained from some more elaborate LC 50 studies (4 h). We determined, besides others, the following pre-screening data (1 h)(in parantheses: values given in ppm): C 2F 5I (>10.000), ICF 2CF 2I (50–100), CF 2ClCFClI (50–100), CCl 2BrCClBrH (100–300), C 3CFICF 3 (1.000–2.000), CF 3CFICF 2Cl (50–100), C 4F 9I (>10.000), C 6F 13,Br (>10.000), C 2F 5CH 2CH 2I (100–500), C 3F 7CH 2CH 2I (250–500), C 4F 9CH 2CH 2I (1.000–5.000), CF 2ClCFCF 2 (<90), (CF 3) 2CCH 2 (500–1.000), BrCF 2CF 2CHCH 2 (>10.000), CF 3CClCClCF 3 (50–100), C 4F 9CHCH 2, (>7.OOO), C 6F 13CHCH 2 (>10.000), CF 2C(CF 3)C(CF 3) CF 2 (100–300), CF 3CH 2OH (>2.500), C 6F 13SOCl (<50), C 2F 5CH 2COCl (100–300), CH 3CH(OCF 2CF 2H)COCl (250–500), C 6F 13CH 2CH 2SH (2.500–5.000). Our results prove that it is possible to predict the order of LC 50 values with surprising accuracy and little experimental effort.