Sera from 18 of 56 individuals working in SLE laboratories bound more than 30 per cent of 0.1 mug of 125-I denatured calf thymus DNA, in an ammonium sulfate precipitation assay. Only two of 58 normal non-laboratory personnel and four of 40 sera from routine hospital laboratory personnel bound more than 30 per cent. Statistical analysis of these results with the Kolmogorov-Smirnov nonparametric test indicated this was a very significant difference (p less than .001). Using native DNA as a test antigen a similar pattern was seen, but it was not as clear cut. Part of this increased reactivity is heat labile. Further studies must be carried out before an adequate interpretation of these findings is possible.