Abstract Lamins are structural proteins found in the fibrous lamina underlining the nuclear envelope. In vitro translation of polyadenylated RNA or polysomes followed by immunoprecipitation with a serum raised against BHK nuclear matrix proteins showed that lamin A (72 kD) is synthesized as a high molecular weight precursor (74 kD) (Laliberté et al., J cell biol 98 (1984) 980) . We have thus investigated the presence in BHK cells of this putative precursor by in vivo labelling with [ 35S]methionine and immunoprecipitation of lamin proteins. Short labelling times, ranging from 5 to 60 min reveal the presence of the 74 kD protein. Pulse-chase experiments indicate that the half-life of the precursor is about 60 min. On two-dimensional gel, the 74 kD protein is resolved in a cluster of isovariants between pH 7.4 and 6.6, which are generally slightly more alkaline than their counterparts in lamin A. These results indicate that lamin A is synthesized as a precursor of 74 kD; the long half-life further suggests that pre-lamin A might accumulate in some sort of cellular pool before undergoing post-transcriptional modification(s) to give the mature form of lamin A.