Abstract: A rapid method for the simultaneous measurement of aggregation or agglutination in plurality of microtest wells or other sample-holding vessels comprises providing a plurality of light beams having discrete focal points adapted to be focused on the contents of a plurality of wells or other vessels and a plurality of photodetectors therefor, directing the light beams through the wells or other vessels and contents thereof, continuously agitating the wells or other vessels and contents thereof with a circular movement effective to permit aggregation or agglutination while preserving the integrity of the material, producing a plurality of output signals from the detectors are proportional to the light absorbencies of the beams as they pass through the wells or other vessels and contents thereof, repeatedly sampling the signals at a predetermined time interval and determining changes in optical densities from the signals obtained after each predetermined time interval by comparing each signal sampled after each p