To measure the flagellar adhesiveness of Chlamydomonas gametes in a more quantitative manner than agglutination assays permit, a binding assay was developed which measured the binding of radioactive flagella of one mating type to unlabeled gametes of the opposite mating type. With the appropriate assay conditions, the number of [3H] flagella specifically bound was shown to be proportional to the number of cells in the incubation mixture and, therefore, to the number of binding sites that were present. The assay was used to study the effects of trypsin treatment on the loss and development of flagellar binding sites. It was shown that after trypsin treatment at least 9 h were required for the return of a full complement of binding sites to the flagellar surface; moreover, the results indicated that these sites reappeared on existing, extended flagella.

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