Polyanions and polycations are known to interact electrostatically and form soluble or insoluble polyelectrolyte complexes. Body fluids, blood and cells are composed of many polyelectrolytic systems such as proteins, glycoproteins, poly(glycosamino glycane)s, polynucleotides, etc. under physiological conditions. Nowadays synthetic polyelectrolytes are proposed as carriers of bioactive compounds, such as drugs and genes, and are thus to be injected into body fluids. For the sake of better understanding the complex behaviour of such artificial polyelectrolytic systems in the pool of natural polyelectrolytes forming living systems, interactions of bi- and multi-components mixtures of synthetic polyanions with the same synthetic polycation, namely poly[(dimethylaminoethyl) methacrylate], HCl, were investigated under the conditions imposed by physiological media, namely pH = 7.4, ionic strength mu = 0.15 and T = 37 degrees C. The selected artificial polyanions were the sodium salts of poly(acrylic acid), poly(methacrylic acid), poly(L-lysine citramide) and poly(styrene sulfonic acid) which have different acid strength, charge density and ionogenic group. The influence of ionic strength and pH on complex formation and stability was investigated by turbidimetry at lambda = 520 nm. Phase separation occurred regardless of ionic strength in the case of sodium polystyrene sulfonate. For the other polyanions, redissolution was observed at critical NaCl concentrations much higher than the physiological ionic strength. In the case of mixtures of two or three polyanions with the polycation, the complex formation appeared polyanion-selective at physiological ionic strength. Data are discussed with regard to phenomena that can occur in vivo.