Intracellular membranes isolated from fully grown immature oocytes, mature oocytes (eggs) and activated eggs ofXenopus laeviswere fractionated through a discontinuous sucrose density gradient into light, intermediate and heavy fractions. Electron microscopy showed that the light and intermediate fractions consisted mainly of smooth membranes, while the heavy fraction consisted mainly of rough membranes and mitochondria. Variations in the proteins associate with samples taken at different stages were observed by SDS-PAGE. The following differences were consistently observed: a 200 kDa protein was present only in the intermediate fraction of activated eggs, 29 and 44 kDa proteins were present only in the intermediate fractions of immature oocytes and activated eggs, and 120 and 145 kDa proteins were present only in the heavy fractions of mature oocytes and activated eggs. Examination of Western blots showed that cyclins A and B₂did not associate with membrane fractions at any stage of meiosis. Instead, cyclin A was present in the cytosols of mature oocytes and cyclin B2 was present in the cytosols of immature and mature oocytes.c-mosprotein was detected in the cytosols and occasionally in the light fractions of mature oocytes and activated eggs. While α- and β-tubulins were detected in the light and intermediate fractions at all the stages of meiosis examined, only β-tubulin was present in the heavy fraction. βtubulin present in the heavy fraction was detected only at interphase, i.e. in immature oocutes and activated eggs, and not in mature oocytes. Using immunogold labelling we confirmed these results and found evidence to suggest that β-tubulin associates with the rough endoplasmic reticulum of interphase cells by a linking protein.