A simple and sensitive turbidimtric immunoassay (TIA) has been described. The automated method has been based on the use of the LKB reaction rate analyser, an instrument that is already present in many clinical chemical laboratories. The precipitin reaction is accelerated and enhanced by poly-ethylene glycol 6000 at a concentration of 50 g/l. Sample volume 2--100 micronl. Use of antiserum: 3--15 micronl per test. Sampling rate 30--60 samples per hour. The choice and the usefulness if antisera is discussed. The major advantage of the present method is the individual measurement and compensation for the initial absorbance (340 nm) immediately after the mixing of antigen and antiserum. This enables the determination of very low serum protein concentrations (from 0.005 mg/ml) which is not possible with other methods that require separate blank determinations. The turbidity course is different in antigen and antibody excess. The The chance of misinterpretation is small. The precision of the method is satisfactory. Within-run precision is 1--2% (C.V.) in the IgA/anti-alpha-system. Between-run-precision in the IgM/anti-micron system: 3.9% (C.V.). The day-to-day precision for IgG, IgA and IgM determinations resulting from a 7-month period of quality control was found to be 6.1% and 10.5%, respectively. RID and TIA methods agree very well. IgM determinations in cord serum from 104 newborns revealed a mean value and standard deviation of 14.6 +/- 6.3 I.U./ml (0.13 +/- 0.06 mg/ml).