An in vitro system is described allowing turbidimetric measurements of calcium phosphates (or other sparingly soluble salts) formed by one-dimensional double diffusion in an agar gel matrix. The basic conditions were investigated over a wide concentration range of calcium and phosphate, and characterized by IR spectra, transmission electron microscopy and selected parameters of the turbidity curve, i.e. the maximal turbidity, the life time of the amorphous calcium phosphate, the optical dispersion and the crystallization rate. The system was also applied to well known effectors, like F-, Mg2+, pyrophosphate and citrate ions as well as two compounds of biological matrices, hyaluronidate and chondroitin-4-sulphate. The results agree well with those observed in aqueous solutions, showing that the overall process in the gel matrix is identical. The economical and efficient method is suitable for large-scale kinetic determinations of compounds affecting nucleation, precipitation and transformation processes of calcium phosphates and other precipitating systems.