Cytofluorimetric analyses with monoclonal antibodies were used to follow the specific effect of highly purified human leucocyte interferon (IFN-alpha) on the expression of transplantation antigens (HLA)1 in Molt 4, a thymus leukaemia cell line. After a lag period of approximately 10 h the amount of HLA steadily increases to reach a maximum at day six of approximately 10 times its original. The effect is reversible and due to an increase in the rate of synthesis of HLA and beta 2m molecules as demonstrated by analysis of [35S]methionine incorporation into specific proteins after a 3-h pulse of Molt 4 cells treated with IFN-alpha for one or six days. A dramatic increase in the amount of mRNA that hybridised with a [32P]cDNA probe containing HLA sequences has also been demonstrated. Surface iodination of IFN-treated Molt 4 cells showed an increase in the iodinated HLA and beta 2m chains and in addition another, apparently unrelated, component with an apparent mol. wt. of 16 000. This component, which appears after incubations with IFN-alpha for longer than 1 h at 37 degrees C, was also detected in the B lymphoid cell lines RAJI and DAUDI.