A procedure has been worked out for the micro estimation of calcium in various types of biological samples with o-cresolphthalein complexone as an indicator. This reagent is more sensitive than the other commonly used indicators such as eriochrome black T and murexide. Test solutions containing 2–5 μg calcium can be conveniently dealt with and the method may have a special application in the analysis of radio-contaminated samples. Though strontium and barium also react selectively with this reagent, their effect is negligible because these two elements normally occur in traces. On the other hand, the most significant effect is due to the presence of magnesium. This interference has been overcome by the use of 8-quinolinol and the method is thereby rendered simple and direct. Results obtained are comparable to those found by the usual standard procedure. In contrast to the method applicable directly to biological fluids such as serum, urine, sweat, milk, and water, an additional step involving prior ashing is required in the case of plant materials. The application of this method may thus be extended further.