Influence of rickets, Ca from diet and different doses of vitamin D analogs on cartilage type II collagen surface charge was studied in experiments on chickens. Chicken cartilage collagen pi were established. It was shown that rachitic type II collagen molecule had more negative charge then normal one. This effect wasn't compensated with Ca load at the absence of vitamin D₃. Influences of vitamin D₃ hydroxy-analogs and maximal dose of 3β-fluorovitamin D₃ were analogous to its effect.