** Adecco ADEN.S shares plummet over 13% with analysts pointing to the group's gross margin coming under pressure again in the first quarter
** The gross margin was 18.8%, down 40 basis points year-on-year, which the group said reflected its current business mix
** "Gross margin is below 4Q guidance of sequential stability," J.P.Morgan says
** Jefferies notes that while top-line beat estimates it was offset by weaker-than-expected gross margin and higher-than-expected SG&A
** Selling, general and administrative expenses were higher than expected at 935 million euros
** Top-line first-quarter results beat analysts' expectations and organic growth accelerated
** Adecco shares extend year-to-date losses which until Tuesday's market close reached nearly 21%
** Stock on track for its worst day since 2004, when it closed 35.2% lower
** Peer Randstad RAND.AS falls over 6% in morning trade
(Reporting by Bernadette Hogg)
((bernadette.hogg@thomsonreuters.com))