A counterimmunoelectrophoresis (CIEP) technique has been developed for the rapid, simple, specific detection of nucleic acids as antigens, or for the detection of precipitating antibodies to nucleic acids or nuclear antigens. The majority of precipitins could be detected within 1 hr. As little as 0·0015 μg of antigen per ml (e.g. poly A: poly U) could be detected. Specificity of rabbit antisera to nucleic acids was demonstrated by selective reactions using a panel of polynucleotides. 1091 patient sera were examined for precipitins to DNA, single-stranded DNA, nucleoprotein and calf thymus nucleoprotein. Precipitins to DNA were found in 42% of systemic lupus erythematosus sera, 9% of rheumatoid arthritis sera and 4% of juvenile rheumatoid arthritis sera. Results with the CIEP method showed equal sensitivity as results obtained by complement fixation or binding assays, but were more sensitive than double diffusion in agar (Ouchterlony).