The nuclear autoantigen SS-B/La, which is recognized by antibodies in certain autoimmune diseases, is complexed with distinct cellular RNAs and the complex of antigen-RNA profile remains constant throughout the cell cycle. This antigen is prominent in the nucleolus during G1/S phase of the cell cycle. Actinomycin D at a concentration of 0.0005 microgram/ml is capable of abolishing the appearance of SS-B/La in the nucleolus. The amount of immune-precipitable SS-B/La-associated RNAs is reduced by 0.005 microgram/ml actinomycin D treatment. The mechanism for these findings is suggested to be related to inhibition of the synthesis of messenger RNA coding for SS-B/La.