PACIA, a homogeneous non-radioimmunoassay, has been adapted to the determination of serum alpha 1-fetoprotein. This technique is based on the agglutination of latex particles coated with antibodies to the antigen to be determined. The agglutination is measured by using an optical cell counter designed to count blood cells, to determine the reduction in the number of non-agglutinated particles. Interferences by serum constituents are avoided by coating the particles with the F(ab')2-fragments of th immunoglobulin G fraction of the antiserum. The system is automated, with a sampling rate of 50/h and an incubation time of 26 min. Concentrations used in preparing the standard curve ranged from 1 to 50 microgram/L; analytical recoveries were 93.5 to 98.4%; the correlation coefficient of PACIA with radioimmunoassay, calculated from results on 127 samples, was 0.98; maximum within- and between-assay CVs were 7.4% and 9.6%, respectively.