Three groups of phosphoproteins have been distinguished, basing on the velocity and extent of phosphorylation in platelets stimulated with collagen. pp60~(c-src) constituted the first group; the increase in its phosphorylation was the highest and most rapid (maximal in 30 s after the addition of collagen). pp80/85 and non-identified protein of 65 kDa formed the second group; the increase in their phosphorylation was twice smaller than that of pp60~(c - src), and reached its maximum 60 s after the addition of collagen. pp120, pp72~(sky), and two non-identified phosphoproteins of 90 and 75 kDa constituted the third group; the increase in their phosphorylation was 4-10-fold lower than that of pp60~(c - src) and reached its maximum after 180 s. We conclude that the phosphorylation of pp60~(c - src) is important for the change of shape of platelets, the phosphorylation of pp80/85 and pp65 for the initiation of the formation of aggregates and the phosphorylation of the third group of phosphoproteins for the formation of massive aggregates. This conclusion was supported by using a monoclonal anti-GPIb antibody, which did not inhibit the shape change of platelets and did not inhibit pp60~(c - src) phosphorylation. This antibody inhibited aggregate formation as well as tyrosine phosphorylation of proteins belonging to the second and the third group of phosphoproteins.
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