For the production of co-precipitates, skim milk (0.05 % milk fat) was used. The chemical complex between casein and whey protein was formed by heating of milk at 87°C during 10 min, and at 95°C during 20 min, respectively. The pasteurised milk (80-85°C during 20 sec) was used as a probe. Co-precipitates were obtained from heat-treated milk by coagulation with 2.8 M HCl, 1.4 M H₂SO₄, 2.8 M lactic and 2.8 M acetic acids, respectively. Co-precipitates obtained by acetic acid were rather dry, lax and white, while co-precipitates obtained by H₂SO₄ and HCl, respectively, were a little sticky, compact and had pronounced yellow undertone. Co-precipitates obtained by lactic acid were compact and softer, probably due to higher moisture content (67.32%). As a measure of recoveries of milk nitrogen matter into co-precipitates, sera nitrogen content was used. Milk sera obtained by separation of co-precipitates, had a little nitrogen matter content, regardless of the acid used (0.0440% for HCl; 0.0465% for H₂SO₄; 0.0500% for acetic acid and 0.0464% for lactic acid). Nitrogen matter that remained in sera represents non-protein nitrogen, which represents 4-6% of total nitrogen matter of milk. Utilisation of nitrogen matter was greater when milk was treated at 87°C during 10 min than by pasteurisation of milk. Co-precipitates obtained from heat treated milk (87°C; 10 min) by HCl had 11.61%. Co-precipitates obtained from pasteurised milk had 11.38% nitrogen matter. Nitrogen matter utilisation was greater when lactic acid was used (11.62%).