Russia decided to transfer S-300 surface to air missile (SAM) systems to the Syrian Arabic Republic (SAR).
No western sources for now, only russian: https://www.gazeta.ru/army/news/2018/04/23/11446483.shtml (one of)
This will greatly improve SAR defence systems. Existing SAR middle-range systems are rather old: S-75 and S-125 in most, and some S-200. These are rather good systems: S-75 used in vietnam war and cause notable casualties to US air forces, one of S-200 downed F-117 while Balcan war.
They also show good results in defending last strikes, but their possibilities are shortened comparing to modern systems. They can strike only one target at a time, while S-300 can strike to six.
Also it can be noted, that S-300 is more applicable for striking down hard and fast targets - jets for example. Using it for blocking tomahawks strike is just overkill. Maybe this is some preparations for US carrier strike group arriving.
Good question, though it's not popular to answer these type of questions because people perceive any answer is defending a side, even if we're trying to look at the situation objectively. Mobilization of military or response may happen in other contexts - mobilization of a cyber military or economic response.
The answer depends on how you see the response or lack thereof so far:
People will see what they want to see in this answer.
Has there been a response or a mobilization of their military that might indicate retaliation?
Russia has stated that the U.S. conferred with it and did not cross Russia's "red lines", effectively ratifying the U.S. action while not actually approving of it. The Washington Post reports that:
Russia’s foreign minister said Friday that the U.S. sought out and respected Moscow’s positions in Syria when it launched its air strikes last week.
Lavrov noted that despite the escalating tensions between Moscow and Washington, the U.S. made sure it didn’t harm any Russian personnel and positions during the strikes against the regime of President Bashar Assad following a suspected chemical attack on the town of Douma.
“We told them where our red lines were, including the geographical red lines,” Lavrov told Russian state television. “The results have shown that they haven’t crossed those lines.”
Russia did not respond during the strike with its advanced air defense systems on the ground in Syria.
Russia did apparently decide to sell Russian S-300 surface to air missiles to Assad's regime, (although not its more sophisticated S-400 systems), after previous equivocating purportedly in the face of Western opposition to the sale.
The S-300 sale is potentially a threat to intervention by air in the Syrian Civil War by the U.S. or its allies (the U.K. and France, for example, also participated in the chemical weapons retaliation strike in April of 2018, and Turkish aircraft have intervened in parts of the Syrian Civil War as well, for example, shooting down a Russian jet fighter which was intruding into its air space). But, this was, in part, a response to the general situation which was being contemplated even before the latest U.S. allied strike related to chemical weapons usage.