We present some very preliminary results of the multifrequency microvariability campaign of two BL Lac objects OJ 287 and Mk 421 carried out mainly during four days in March 1993. During these four days also IUE was observing these blazars. In this poster we present only a small part of the whole data: radio observations at Metsähovi, CCD observations on La Palma and Heidelberg and UBVRI photopolarimetric observations at Crimea. Because of the very bad global weather conditions and also because of the faintnes of OJ 287 our optical observations were very limited.