Rabbit skeletal muscle actin was labeled with 5-iodoacetamidofluorescein (5-IAF) and purified by gel filtration, ion-exchange chromatography, and polymerization-depolymerization. The resultant fluorescent conjugates retained full biochemical activities. The labeled actin was incorporated into unfertilized eggs of Lytechinus pictus by direct microinjection and the distribution of fluorescence was investigated after fertilization through the first division cycle. The results were interpreted by comparing the images with those of control eggs injected with fluorescein isothiocyanate (FITC)-labeled ovalbumin. After fertilization of eggs containing IAF actin, the membrane-cortical regions showed dramatic increases in fluorescence intensity which were not observed in FITC ovalbumin controls. During the first division, spindle regions of both IAF-actin-injected eggs and control eggs became distinctly fluorescent. However, no distinctly fluorescent contractile ring was detected in the cleavage furrow. After cytokinesis, the surface between blastomeres containing IAF actin exhibited an increase in fluorescence intensity. These observations have been compared with those of previous studies using different methods, and the possible implications have been discussed in relation to cellular functions.