Abstract The binding of 3H-actinomycin D (AMD) to the SAT-chromosomes (S 1, S 2) of the embryo suspensor cells of Phaseolus coccineus has been analysed after different periods of exposure to photographic emulsion. A differential labelling pattern has been observed in the two pairs S 1 and S 2, the main features concerning the centromere and satellite regions. It has been also found that the AMD binding pattern does not reproduce the distribution pattern of heterochromatin. The results may indicate that labelling detectable at the shortest exposure time makes manifest guanine-rich segments of DNA, while lack of labelling at the longer exposure evidences thymine-rich segments. The possibility that a different amount and/or kind of chromosomal proteins is responsible for the observed DNA response to AMD is also discussed.