The article deals with the circumstances of Roman intervention in the defence of the Ptolemies during the Sixth Syrian War. The author discussed the outbreak of the war and the diplomatic campaigns conducted by both sides in Rome. The Roman attitude towards the conflict was determined by the events in Greece and Macedonia : the outbreak and course of the Third Macedonian War. Faced with wartime defeats, the Ptolemies turned to Rome with a request for assistance. The Romans remained extremely cautious. Unclear source information produced controversies relating to the circumstances of dispatching Popilius Laenas and the instructions with which he was outfitted. The envoy left Rome in January 168, entrusted with the task of initiating mediation between the Ptolemies and the Seleucids, and was supposed to reach Alexandria. The temporary withdrawal of Antiochus IV from Egypt meant, however, that the mission stopped in Greece, from which it observed the development of events in that country. In the face of a renewed attack, the Ptolemies once again sought help in, i. a. Rome, and presumably in a response to their mission the Senate provided Laenas with further instructions, which this time contained an ultimatum (quite possibly secret) to be presented to Antiochus IV. The decisive battle of Pydna, which resolved the Third Macedonian War, provided Rome with an opportunity for a decisive campaign in Egypt; by presenting the Senate's ultimatum at Eleusis, Laenas forced Antiochus IV to withdraw from Egypt.